Generate inline documentation if it's available in the extension_api.json

[dsnopek]

Fixes #604

This uses functionality from PR godotengine/godot#82331, which optionally adds documentation to the extension_api.json, to generate Doxygen-style inline documentation if it's available.

This is marked as a DRAFT because it's incomplete! There's more places to inject the documentation, and there's more bbcode to parse.

This also updates the included extension_api.json to have the documentation. I could see an argument for not including this, since it increases the size of the file by 65%, and then leaving it up to users to include the documentation if they want it. However, I suspect that quite a few folks are going to want it?

[DmitriySalnikov]

That's cool! With this PR, it will finally be possible not to run Godot to find out details about the class and methods being used.

Even 10MB of extension_api.json won't be a problem. This will not affect the size of the binaries, and besides this file there is a lot of garbage that takes up much more space, for example, the v17\ipch\AutoPCH folder from Visual Studio, which takes about 1GB when I open my project.

[Naros]

I most definitely like the idea of this being part of the extension_api.json file. I am already doing a processing pass on this file as-is to generate an ExtensionDB class for quite a bit of metadata that isn't directly accessible from ClassDB, so having access to this documentation will be helpful for my Visual Scripting solution's tooltips and the user experience.

[GoldAndPurple]

What's the priority on this PR?

[dsnopek]

What's the priority on this PR?

It's not a very high priority for me, personally. So, if someone else is super motivated to finish this, feel free to make a new PR to supersede this one! Otherwise, I will get back to it eventually. :-)

[Naros]

I can certainly take a look, but it'll likely be closer to the weekend before I can @GoldAndPurple