Add section on name lookup and shadowing to GDScript Reference page

**Your Godot version:**
4.4
**Issue description:**
See https://github.com/godotengine/godot-docs/pull/10679#discussion_r1957049123 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