C#: Bindings generator now translates BBCode docs to XML comments

[neikeq]

It's not perfect, but much better than what we had before.

Here's the full diff of the generated bindings: https://gist.github.com/neikeq/5298b69567a24a683a42777aa3fcdbb7

Regarding member/type/etc references, right now the following are not automatically resolved but it would be possible with some mapping:

Generating FuncRef.cs...
Cannot resolve type from method reference in documentation: @GDScript.funcref
Cannot resolve type from method reference in documentation: @GDScript.funcref

Generating GDScriptFunctionState.cs...
Cannot resolve type from method reference in documentation: @GDScript.yield
Cannot resolve type from method reference in documentation: @GDScript.yield

Generating Node.cs...
Cannot resolve method reference for non-Godot.Object type in documentation: String.match
Cannot resolve method reference for non-Godot.Object type in documentation: String.match

Generating Texture.cs...
Cannot resolve type from method reference in documentation: @GDScript.load

Generating WeakRef.cs...
Cannot resolve type from method reference in documentation: @GDScript.weakref

[neikeq]

Marking this for cherry-picking, as it's a documentation improvement that would be great to have in the 3.1.x branch.

[akien-mga]

Cherry-picked for 3.1.1.