feat(sidebars): add depth option to listSubPages[Grouped]

[caugner]

Description

Allows to listSubPages recursively.

Motivation

Some CSS functions like color-mix() are nested, so they currently no longer appear in the sidebar, because depth defaults to 1.

Additional details

To test this, apply this change in content:

diff --git a/files/sidebars/cssref.yaml b/files/sidebars/cssref.yaml
index 3f2b20e1f8..a6b0e741d6 100644
--- a/files/sidebars/cssref.yaml
+++ b/files/sidebars/cssref.yaml
@@ -96,6 +96,7 @@ sidebar:
     path: /Web/CSS
     title: Functions
     tags: css-function
+    depth: 2
     details: closed
   - type: listSubPages
     path: /Web/CSS

Related issues and pull requests

Fixes #73.

[caugner]

Unfortunately, this approach doesn't work, because the list_subpages_internal() implementation currently always renders a tree structure, and skips intermediate nodes that don't match the filtered page type.

It doesn't seem trivial to extend the behavior of this function, because it mixes querying and displaying, e.g. sub-trees are individually sorted, so we cannot easily display nested pages in a flat list.