Update UI contributing docs

[bbovenzi]

Add docs on how to run the new airflow/ui project. And add temporary rules during Airflow 3.0 development, specifying when to contribute to the new airflow/ui and airflow/ui_api projects vs the legacy airflow/www UI and endpoints.

---

^ Add meaningful description above
Read the Pull Request Guidelines for more information.
In case of fundamental code changes, an Airflow Improvement Proposal (AIP) is needed.
In case of a new dependency, check compliance with the ASF 3rd Party License Policy.
In case of backwards incompatible changes please leave a note in a newsfragment file, named {pr_number}.significant.rst or {issue_number}.significant.rst, in newsfragments.

[kaxil]

Minor details. Worth dropping a note on the dev mailing list too to make sure committers have seen it.

[potiuk]

Nice. Other than the comments from @jscheffl and @kaxil -> no more from me.

[jscheffl]

Made local setup and tested... works. Got an empty UI now :-D