Add a gdextension online documentation guide.

[Ivorforce]

First step to address https://github.com/godotengine/godot-proposals/issues/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.

[Ivorforce]

Forgot that I also needed version.py. i added that bit in.

[mhilbrunner]

Thank you! Merged.

[mhilbrunner]

Cherrypicked to 4.3 in https://github.com/godotengine/godot-docs/pull/10346.