Add Sphinx-Gallery caching and stabilize CI cache keys

[bruAristimunha]

Summary

• Add Sphinx-Gallery output caching in docs.yml keyed by hashFiles('examples/**/*.py'), so unchanged examples skip re-execution on subsequent builds
• Stabilize data cache keys in both docs.yml and install-tests.yml by replacing github.run_id with a fixed v1 suffix — downloaded datasets (MNE, Nilearn) don't change between runs, so the run_id-based key was creating unnecessary cache entries

Context

Current docs build on main takes ~23 min. PR #21 adds a heavy example (plot_liebn_batch_normalization.py) that causes builds to hang for 6+ hours. This PR provides caching infrastructure so that once examples are built, they won't re-run unless the example scripts change.

Test plan

• Verify docs CI completes successfully
• On a second push with no example changes, verify gallery cache is restored (check "Restore Sphinx-Gallery Cache" step output)

[github-actions]

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