Scene inheritance is currently undocumented

**Your Godot version:**
4.0.dev
**Issue description:**
There's currently very little documentation about Scene Inheritance. The few references I could find were mostly tutorial subtopics [like here](https://docs.godotengine.org/en/stable/getting_started/step_by_step/ui_game_user_interface.html#inherit-the-bar-scene-to-build-the-lifebar) [or here](https://docs.godotengine.org/en/stable/getting_started/workflow/assets/importing_scenes.html#scene-inheritance), and I think the second one is incorrect, as in my tests, sub-resources still can be edited in inherited scenes, but they are shared. This could be something specific to 3D modelling that I'm not aware of.

Besides that, when looking up "Scene Inheritance" on google the first page is mostly user questions about it, with no official resource in sight. Compare this with the similar Instancing feature, which has it [own page](https://docs.godotengine.org/en/stable/getting_started/step_by_step/instancing.html), which pops up as the first result on google.

I think inheritance could use the same treatment and get its own page in the "step by step", or this hypothetical page could be merged with the Instancing one, with a comparison of when either method is desirable, and possibly something describing their relationship with Make Unique and Local to Scene.

**URL to the documentation page (if already existing):**
N/A