Cache .repository folder and simplify use of pip cache

[tomaskontrimas]

Rerunning the job seems to have restored the cache successfully:

Cache artifacts be found here: https://github.com/icecube/skyllh/actions/caches

The only caveat is that when we change the (expected) content of .repository we should remove the cached folder in actions/caches or increment the version key so that the cache is updated.

[Copilot]

Pull request overview

This PR updates the GitHub Actions workflows to rely on actions/setup-python’s built-in pip caching and adds caching for the local .repository dataset folder to speed up repeated CI runs.

Changes:

• Switch documentation workflow from a custom pip cache dir + actions/cache setup to actions/setup-python’s cache: pip.
• Enable pip caching in the CI pytest matrix via actions/setup-python.
• Add an actions/cache step to persist the .repository folder between CI runs.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
.github/workflows/documentation.yml	Simplifies dependency caching by using setup-python’s pip cache and streamlines the install step.
.github/workflows/ci.yml	Enables pip caching and introduces caching of .repository to reduce repeated dataset setup work across CI runs.

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[tomaskontrimas]

I think it's fine as it happens only during the run with the first cache miss and I don't want to create another job to download to the cache first