Split generated RST for class reference based on the base type

[YuriSizov]

Related to godotengine/godot-proposals#4543.

Splits generated RSTs by using a different file prefix based on the base type of each class (node/resource/other), plus a dedicated group for global scopes (starting with @). This works in tandem with the godot-docs PR (godotengine/godot-docs#5990) to give this kind of grouping:

chrome_2022-07-26_18-59-21.mp4

[akien-mga]

I like the idea generally, but I see a few issues:

• "Nodes" / "Resources" / "Classes" - the last one is a weird catch-all. Nodes and Resources are classes too.
• Doing the split based on adding prefixes means that we break URLs for all existing classes, and we'll have to change :ref:s all over the tutorials when referring to a class. Now one would need to remember if the class is a Node, a Resource or another "Class" to use the correct ref prefix.

[YuriSizov]

the last one is a weird catch-all

It is, I kept it like that to avoid breaking all the links. Well, I guess the section itself can be called whatever... Objects, maybe? But I think from a casual user's perspective it's quite clear even with the current split.

we'll have to change :ref:s all over the tutorials when referring to a class.

No, internal identifiers for the classes are still the same and all the references should continue to work.

we break URLs for all existing classes

Yes, that is true. But this is for 4.0+, where a lot of old googled links break already due to renames, right? That said, if we change existing class_* prefix in files to something else, then we can generate class_* files with redirects or notes for every class.

[akien-mga]

We discussed this in a meeting, the feature makes sense.

We decided to try to keep the existing format for the URL to avoid the issue of having to know how a given class was categorized to guess its URL - several of us tend to go to the class reference for a given class by editing the URL manually in our browser. This might also be relied upon by external tools that want to link to the Godot docs programmatically.

To preserve this feature, we decided to have make_rst.py generate the index.rst with the subcategories instead of relying on prefixes.

[YuriSizov]

Changes done, the index.rst file is now generated alongside the rest of them, with a hardcoded list of all classes, split into groups as before.

Compared to the original version of this PR I've renamed "Classes" to "Objects", as it makes a bit more sense IMO. The rest should be functionally the same. Looks a little something like this:

[akien-mga]

It looks amazing! 🎉

[akien-mga]

Thanks!