[Docs] Update layer documentation (qmk#8371)

* [Docs] Update layer documentation

* Add layer_state_cmp functions

* Fix cut/copy/paste issue

* Add id tags

* Apply noroads corrections

* Move Layers section to separate document

* Fix ID tag for layers

* Use better name for summary/side bar

* Fix feature page linkage

As well as a small spell error close by

* Remove paper analogy for now

docs/_summary.md

@@ -52,7 +52,7 @@
   * Simple Keycodes
     * [Full List](keycodes.md)
     * [Basic Keycodes](keycodes_basic.md)
-    * [Layer Switching](feature_advanced_keycodes.md)
+    * [Modifier Keys](feature_advanced_keycodes.md)
     * [Quantum Keycodes](quantum_keycodes.md)
 
   * Advanced Keycodes
@@ -71,6 +71,7 @@
     * [Combos](feature_combo.md)
     * [Debounce API](feature_debounce_type.md)
     * [Key Lock](feature_key_lock.md)
+    * [Layers](feature_layers.md)
     * [One Shot Keys](one_shot_keys.md)
     * [Pointing Device](feature_pointing_device.md)
     * [Swap Hands](feature_swap_hands.md)

docs/feature_advanced_keycodes.md

@@ -1,46 +1,3 @@
-# Switching and Toggling Layers :id=switching-and-toggling-layers
-
-These functions allow you to activate layers in various ways. Note that layers are not generally independent layouts -- multiple layers can be activated at once, and it's typical for layers to use `KC_TRNS` to allow keypresses to pass through to lower layers. For a detailed explanation of layers, see [Keymap Overview](keymap.md#keymap-and-layers). When using momentary layer switching with MO(), LM(), TT(), or LT(), make sure to leave the key on the above layers transparent or it may not work as intended.
-
-* `DF(layer)` - switches the default layer. The default layer is the always-active base layer that other layers stack on top of. See below for more about the default layer. This might be used to switch from QWERTY to Dvorak layout. (Note that this is a temporary switch that only persists until the keyboard loses power. To modify the default layer in a persistent way requires deeper customization, such as calling the `set_single_persistent_default_layer` function inside of [process_record_user](custom_quantum_functions.md#programming-the-behavior-of-any-keycode).)
-* `MO(layer)` - momentarily activates *layer*. As soon as you let go of the key, the layer is deactivated. 
-* `LM(layer, mod)` - Momentarily activates *layer* (like `MO`), but with modifier(s) *mod* active. Only supports layers 0-15 and the left modifiers: `MOD_LCTL`, `MOD_LSFT`, `MOD_LALT`, `MOD_LGUI` (note the use of `MOD_` constants instead of `KC_`). These modifiers can be combined using bitwise OR, e.g. `LM(_RAISE, MOD_LCTL | MOD_LALT)`.
-* `LT(layer, kc)` - momentarily activates *layer* when held, and sends *kc* when tapped. Only supports layers 0-15.
-* `OSL(layer)` - momentarily activates *layer* until the next key is pressed. See [One Shot Keys](one_shot_keys.md) for details and additional functionality.
-* `TG(layer)` - toggles *layer*, activating it if it's inactive and vice versa
-* `TO(layer)` - activates *layer* and de-activates all other layers (except your default layer). This function is special, because instead of just adding/removing one layer to your active layer stack, it will completely replace your current active layers, uniquely allowing you to replace higher layers with a lower one. This is activated on keydown (as soon as the key is pressed).
-* `TT(layer)` - Layer Tap-Toggle. If you hold the key down, *layer* is activated, and then is de-activated when you let go (like `MO`). If you repeatedly tap it, the layer will be toggled on or off (like `TG`). It needs 5 taps by default, but you can change this by defining `TAPPING_TOGGLE` -- for example, `#define TAPPING_TOGGLE 2` to toggle on just two taps.
-
-## Caveats
-
-Currently, `LT()` and `MT()` are limited to the [Basic Keycode set](keycodes_basic.md), meaning you can't use keycodes like `LCTL()`, `KC_TILD`, or anything greater than `0xFF`. Modifiers specified as part of a Layer Tap or Mod Tap's keycode will be ignored. If you need to apply modifiers to your tapped keycode, [Tap Dance](feature_tap_dance.md#example-5-using-tap-dance-for-advanced-mod-tap-and-layer-tap-keys) can be used to accomplish this.
-
-Additionally, if at least one right-handed modifier is specified in a Mod Tap or Layer Tap, it will cause all modifiers specified to become right-handed, so it is not possible to mix and match the two.
-
-# Working with Layers
-
-Care must be taken when switching layers, it's possible to lock yourself into a layer with no way to deactivate that layer (without unplugging your keyboard.) We've created some guidelines to help users avoid the most common problems.
-
-## Beginners
-
-If you are just getting started with QMK you will want to keep everything simple. Follow these guidelines when setting up your layers:
-
-* Setup layer 0 as your default, "base" layer. This is your normal typing layer, and could be whatever layout you want (qwerty, dvorak, colemak, etc.). It's important to set this as the lowest layer since it will typically have most or all of the keyboard's keys defined, so would block other layers from having any effect if it were above them (i.e., had a higher layer number). 
-* Arrange your layers in a "tree" layout, with layer 0 as the root. Do not try to enter the same layer from more than one other layer.
-* In a layer's keymap, only reference higher-numbered layers. Because layers are processed from the highest-numbered (topmost) active layer down, modifying the state of lower layers can be tricky and error-prone.
-
-## Intermediate Users
-
-Sometimes you need more than one base layer. For example, if you want to switch between QWERTY and Dvorak, switch between layouts for different countries, or switch your layout for different videogames. Your base layers should always be the lowest numbered layers. When you have multiple base layers you should always treat them as mutually exclusive. When one base layer is on the others are off.
-
-## Advanced Users
-
-Once you have a good feel for how layers work and what you can do, you can get more creative. The rules listed in the beginner section will help you be successful by avoiding some of the tricker details but they can be constraining, especially for ultra-compact keyboard users. Understanding how layers work will allow you to use them in more advanced ways.
-
-Layers stack on top of each other in numerical order. When determining what a keypress does, QMK scans the layers from the top down, stopping when it reaches the first active layer that is not set to `KC_TRNS`. As a result if you activate a layer that is numerically lower than your current layer, and your current layer (or another layer that is active and higher than your target layer) has something other than `KC_TRNS`, that is the key that will be sent, not the key on the layer you just activated. This is the cause of most people's "why doesn't my layer get switched" problem.
-
-Sometimes, you might want to switch between layers in a macro or as part of a tap dance routine. `layer_on` activates a layer, and `layer_off` deactivates it. More layer-related functions can be found in [action_layer.h](https://github.com/qmk/qmk_firmware/blob/master/tmk_core/common/action_layer.h).
-
 # Modifier Keys :id=modifier-keys
 
 These allow you to combine a modifier with a keycode. When pressed, the keydown event for the modifier, then `kc` will be sent. On release, the keyup event for `kc`, then the modifier will be sent.
@@ -63,10 +20,14 @@ These allow you to combine a modifier with a keycode. When pressed, the keydown
 
 You can also chain them, for example `LCTL(LALT(KC_DEL))` makes a key that sends Control+Alt+Delete with a single keypress.
 
-# Legacy Content
+# Legacy Content :id=legacy-content
 
 This page used to encompass a large set of features. We have moved many sections that used to be part of this page to their own pages. Everything below this point is simply a redirect so that people following old links on the web find what they're looking for.
 
+## Layers :id=switching-and-toggling-layers
+
+* [Layers](feature_layers.md)
+
 ## Mod-Tap :id=mod-tap
 
 * [Mod-Tap](mod_tap.md)