Add a gdextension online documentation guide.

[Ivorforce]

First step to address godotengine/godot-proposals#10733.

The guide explains, in very short, how to create online documentation (much like the godot docs themselves) for a gdextension.

You can see a working example here: https://numdot.readthedocs.io/ (source).

A link to my example might be helpful to recreate what I did - but I elected not to link it because I felt it may be weird to link to a third party project. Let me know what you think.

[AThousandShips]

Thank you for your contribution!

You should target master not 4.3, changes can then be cherry picked, see here.

Please open a new PR and a new branch based on the correct branch, if needed (check the latest version first).

[Calinou]

• Superseded by Add a gdextension online documentation guide. #9966.