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.

[paddy-exe]

Looks good so far. Some parts could be phrased differently to make it clearer how to set it up though. Thanks for contributing to the docs 🙌🏻

[paddy-exe]

I think this should be phrased differently. Many of the people reading the docs will try to mimic the instructions even with a different setup so this should be phrased that the setup should be identical so the steps work. Otherwise we will get open issues complaining that the steps don't work.

[Ivorforce]

I'm trying to convey that the steps are very 'unprecise' with this sentence, because I needed to use a good amount of insight (while editing those 3 files) beyond this simple plan to actually get it to run. Describing all of the steps here would result in a pretty large and probably unhelpful and soon to be outdated tutorial.

The only assumption the text is making is that the documentation files are in doc_classes (ensured by doctool). It should really work with any kind of setup beyond that.

I'm not sure how to phrase it differently to convey this better, got any suggestions?

[paddy-exe]

@Ivorforce Perhaps along the lines of:

The setup may differ depending on how your project is structured. Here are the exemplary steps you would need to take to compile the documentation into rst files.

[Ivorforce]

I think i see where you're coming from, but the following steps actually do not depend on the project structure at all. This is just a guideline for how to adapt the Godot docs with different content. It's basically independent of the project itself, at least after the XML Generation step.

[Ivorforce]

I have edited the section with a new phrasing. Hopefully this conveys it better?

[Ivorforce]

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