Simplify the "Adding documentation" article for godot-cpp, and de-duplicate format information.

[Ivorforce]

The current documentation for documentation in godot-cpp covers everything that's necessary, but there are several things I'd like to improve:

• The information about version compatibility is too extensive, and distracts from important information. People will either use 4.3 or newer, and read the article, or use 4.2 or lower, and skip it via the disclaimer.
• The disclaimer about assumptions of the article (e.g. with the file structure) is not needed. The actual assumptions we need are just that there is a demo folder. Listing more assumptions than needed distracts from the contents, and might unnecessarily scare away potential readers.
• The Documentation styling section is duplicate to the "Class Reference Primer". The format used by Godot's class reference is the exact same as it is for GDExtension. We do not need this information twice.
• I adjusted wording to the new structure, and hopefully made it easier to read.
• I've also added a link to the documentation guidelines. While it's written for potential contributors, it's also a good reference for writing docs for GDExtensions.