Document local virtualenv setup with workspaces

[potiuk]

With #42505 and few follow-up issuse we've worked out how to use uv workspaces, to have the source tree split between airflow and providers. This means that it is possible to have airflow and providers development working fot both breeze and local virtual environment using uv workspaces

While with Breeze it works out-of-the-box, the local virtualenv setup and IDE setup for IntelliJ, VS Code requires some adjustments. Our documentation still describes how it works for the "pre-providers-split" and our contributors (both new and those who already contributed) are a bit confused what to do and how to set-it up - because there is not even a mentioning of uv and workspaces in the contributing documentation.

While a number of people who use the workspaces already had worked out their patterns, this is not explained and knowledge about it is not shared with other contributors, especially that uv workspace is a relatively new feature and by far not something we can assume that people know about.

We need to update https://github.com/apache/airflow/blob/main/contributing-docs/03_contributors_quick_start.rst https://github.com/apache/airflow/blob/main/contributing-docs/07_local_virtualenv.rst and related IDE guides in those chapters to explain it better.