Extension is not documented

[YuriSizov]

Right now the GDSiON extension provides only the auto-generated documentation, containing method and property names and types, but no functional description or guidance.

GDExtensions may not support XML documentation yet, so in the meantime any other means to describe the functionality should be explored, including Markdown files and GitHub wiki.

The original SiON project has quite a lot useful comments with descriptions and examples, which can be lifted as a starting point. Note that some liberties were taken when porting certain aspects of the original implementation, so care must be applied when borrowing descriptions. I also noticed a few outdated comments/errors/copy-paste mistakes while working on the codebase.

[YuriSizov]

Some points of reference:

• Editor support for XML docs added in January: GDExtension: Add an interface for loading extra documentation godotengine/godot#83747
• godot-cpp support is still missing: Allow submitting documentation to the Godot editor godotengine/godot-cpp#1374
• It may be possible to do some changes on the extension side and still make it work with Godot 4.3: Riteo/threen@bba92bb#diff-442e6baf64aeb36da06077ebdd82f4e17d1a80fb2870456683a2ba19828f33c0