De-duplicate BBCodes information

Referring to https://github.com/godotengine/godot-docs/pull/9996#issuecomment-2373129310, BBCodes are currently documented ~~3~~ ~5~ 4 times:

- [GDScript documentation comments](https://docs.godotengine.org/en/latest/tutorials/scripting/gdscript/gdscript_documentation_comments.html#bbcode-and-class-reference)
- ~[GDExtension documentation system](https://docs.godotengine.org/en/latest/tutorials/scripting/gdextension/gdextension_docs_system.html#documentation-styling)~
- [Class reference primer](https://docs.godotengine.org/en/latest/engine_details/class_reference/index.html)
- [RichTextLabel](https://docs.godotengine.org/en/stable/tutorials/ui/bbcode_in_richtextlabel.html)
- [Class Reference Translation](https://docs.godotengine.org/en/stable/contributing/documentation/editor_and_docs_localization.html#class-reference-bbcode)

It might be better to have a separate, independent page for bbcodes. If the info from RichTextLabel should not be merged into the ones for the documentation system, at least the others can be merged together as they really are the same use-case.