Add and expose a method for getting a list of script documentation in the Script class

[Xwdit]

Replace #62928 since I messed something up in the original PR.

Part modified from #39849 by @ThakeeNathees

This allows users to access script documentation through scripts when tools=yes, making it easy to export script documentation to xml/json/bbcode/markdown etc, or create any tool based on it.

Partial implementation godotengine/godot-proposals#177

This should allow more freedom than exporting several preset document formats through the editor UI, allowing community to easily create various addons to export documents in different formats.

For example, here is a Json/Markdown documentation generator based on this PR: Script Documentation Generator.zip

@akien-mga @YuriSizov

[Xwdit]

Will this be moved to the 4.x milestone even though the original PR was submitted before the feature freeze?

[YuriSizov]

Will this be moved to the 4.x milestone even though the original PR was submitted before the feature freeze?

4.x just means it can be merged in any 4.x version. If this is approved soon, it can be included in 4.0. But as it stands, it doesn't block 4.0, I don't think.

[mhilbrunner]

I'm not a fan of enshrining this structure in a (for us) hard to find place we need to keep updated. It also tends to change, so we'd need to maintain it. Maybe we could link to doc_data.h?

[YuriSizov]

I guess #76490 supersedes this as a more practical solution. It doesn't expose any scripting API, but instead if generates XML in the same format that the engine uses. Thanks for your contribution nevertheless!