Document best practices for maintaining the cache

[zanieb]

e.g. as prompted in #5222 (comment)

How to remove old entries? (uv cache prune?)
How to remove specific packages? (uv cache clean <name>)
How to optimize size in CI?

[zanieb]

See also:

• Question: caching venv vs caching uv? hynek/setup-cached-uv#7
• Support for uv Package Manager Caching actions/setup-python#822

[zanieb]

See also:

• Add --ci mode to uv cache prune #5391 (comment)

[MarcusJellinghaus]

Maybe you also want to add information related to

• running several CI pipelines in parallel (eg several GitHub runners on one Windows machine) - should I have one cache for all or several caches?
• incorrect treatment of new packages - maybe also add this comment
• best practices for running CI pipelines on Windows servers ( i.e. where the CI server might stay the same for years, while kubernetes-based pods might get recreated from template regularly).