Add note within ux documentation that further changes should be made within the…

What does this MR do?

Adds note to UX guide regarding deprecation of existing docs and instructions for how to make changes during the transition.

Why was this MR needed?

Documentation is being transferred to design.gitlab.com. If changes are made to the documentation here after the content was already transferred, we could lose the updates/additions in this transfer.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• [-] API support added
• [-] Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • [-] Has been reviewed by Frontend
  • [-] Has been reviewed by Backend
  • [-] Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-qa manual pipeline job)