Add section on name lookup and shadowing to GDScript Reference page

[tetrapod00]

Your Godot version:
4.4
Issue description:
See #10679 (comment) for original context.

• Add a short section on how name lookup and shadowing works.
• Possibly mention named classes, possibly named classes section links to this new section
• Mention the warning you get when shadowing a name.

URL to the documentation page (if already existing):
https://docs.godotengine.org/en/stable/tutorials/scripting/gdscript/gdscript_basics.html