ci(docs): fix stale cache key and add a downloadable PR docs-build artifact

[pellet]

The doc-build cache key only hashes examples/**/*.py, doc/**/*, conf.py. A change to library code under eegnb/ (e.g. #330) still trips the "full build" changed-files check, but the cache step restores the same doc/_build/html regardless — its key hasn't moved — and sphinx-gallery skips re-running any example script that's itself unchanged. Net effect: this CI can report green without ever re-rendering the example the library change actually affects.

Widening the key isn't enough on its own. restore-keys: ${{ runner.os }}-sphinx- prefix-matches the previous entry, so a miss on the exact key still restores the old doc/_build/html and sphinx-gallery still skips. Dropping restore-keys would close that, but it would also throw away the incremental case the Determine build mode step exists to serve — an examples-only PR would then rebuild every example from cold. So this keeps the fallback and instead deletes the restored HTML when FULL_BUILD=true, which is already the workflow's own name for "something other than an example changed", and is the same flag doc/conf.py reads to widen sphinx-gallery's filename_pattern. The wider cache key still earns its place: it keeps the saved entry tracking eegnb/ instead of pinning it to a hash that ignores library code.

Also: there's currently no way to see a PR's rendered docs short of checking it out and running make docs locally — docs.yml only publishes to GitHub Pages on push to master. This adds actions/upload-artifact so every run's built HTML is downloadable from the PR's checks tab.

No behavior change on master pushes (still deploys to Pages as before).

Related: #322 (draft) also adds an upload-artifact step to this workflow and goes further, deploying a live rendered preview to gh-pages/pr-preview/. This PR deliberately stops at the artifact: the preview push needs a write token, which pull_request runs from forks don't get, so a downloadable artifact is the part that works on every PR. The cache-key fix is orthogonal and isn't covered by #322.