docs: guard cache usage in cache fixture example

[kartikdp]

Summary

This fixes a docs bug in the cache example.

The example in doc/en/how-to/cache.rst accessed pytestconfig.cache
directly, which can raise AttributeError when the cacheprovider plugin is
disabled (for example, with -p no:cacheprovider).

The fixture example now:

• gets cache safely with getattr(pytestconfig, "cache", None)
• skips when cache is unavailable
• uses the local cache variable for get/set

closes #14148

Impact

• Prevents runtime failure from a copy-pasted docs example in valid pytest
  configurations.
• Improves correctness of official docs with minimal, docs-only change.
• No behavior change to pytest runtime code.

How tested

• uv run --with tox tox -e docs
• Docs build passed with warnings as errors.

Checklist

• Include documentation when adding new features. (Docs updated for this
  behavior)
• Include new tests or update existing tests when applicable. (Not
  applicable: docs-only fix)
• Allow maintainers to push and squash when merging my commits.
• Add text like closes #XYZW to PR description and/or commits.
• If AI agents were used, they are credited in Co-authored-by commit
  trailers.
• Create a new changelog file in changelog (not required for this small
  docs fix).
• Add yourself to AUTHORS (not required for this small docs fix).

[kartikdp]

All checks are green; ready for review.

[kartikdp]

Thanks for reviewing. I noticed this overlaps with #14254, which was opened earlier and is already in review, so I’m closing this one as a duplicate.