Document sub plugins

[HolonProduction]

godotengine/godot-proposals#3782 and godotengine/godot-proposals#10556 ask for a way to add sub plugins, which as it turns out is already possible. With this PR it is also a documented feature.

I also made some other small changes:

• fixed Adding autoloads from custom plugin: example in the doc is bad practice #9571
• moved the "Going beyond" section to the very end, adding autoloads is a very basic thing to do in a plugin and shouldn't be placed after it. One could argue that sub-plugins should be placed behind it, but the section feels more like a closing to the whole page. (The complete plugin section might need a little restructuring at some point though, we have installing plugins then multiple pages named as "making ... plugins" and then some named just as "... plugins" which also talk about making plugins, so they should either be renamed, or "making plugins" should become a collection of pages with everything related to making plugins below it.)
• Upgraded "A custom panel" to a real heading, since it isn't a subsection of "A custom node"

[mhilbrunner]

Thank you! Merged. Oof, this took a bit, sorry :)