Rewrite of `Game Studio`

This is something I have been working for the past few months and it seems like it's not gonna be finished any time soon, so I'm creating this issue to keep track of this. I have only learned that you are meant to that after trying to change the contributing tab 😅

Motivation

Having Assets and Scenes be under Game Studio provides a lot of limitations when writing new pages and make it seem like the documentation for certain features (e.g. scene loading) is missing. On top of that, actual Game Studio documentation such as all of it's panels, settings and features are undocumented or scattered across random pages.

Proposed changes

• Move Game Studio/Scenes to Scenes.
• Move Game Studio/Assets to Assets.
• Move Game Studio/Prefabs to Scenes/Prefabs.
• Move Game Studio/Game Settings to Project/Game Settings.
• Move Game Studio/World Units to somewhere in Scenes.

This leaves Game Studio without a single page, which leaves space for new content:

• Add a page for every panel.
• Add a page for every dedicated editor - these should only document their features and link to specific instructions on how to use them to other sections.
• Add a page for settings - goes over every option and what it does, linking to additional pages if necessary.

While moving Game Studio's current categories outside, most of the pages should be updated and rewritten with a better structure that can support future content.

Tasks

Originally, I tried to split the work into multiple tasks, but the problem is that most of these pages rely on each other. The only exception is the Assets section (https://github.com/stride3d/stride-docs/pull/501).

Start by inventorying the existing Game Studio documentation and comparing it with the proposed category moves. Review the Assets work in pull request #501, then map the remaining panels, editors, settings, and dependent pages. Done means the categories are reorganized and the Game Studio coverage is complete without scattered or missing documentation.