Resolve "Follow-up from "Resolve "Native Apple silicon support"""

What does this Merge Request do and why?

This MR improves the documentation around macOS arm64 installs and the new .gdkrc.custom file.

Merge Request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise Issue to follow-up.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.

Closes #1410 (closed)