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

The doc build cache key only hashed examples/**/*.py, doc/**/*, and
conf.py. A change to library code under eegnb/ (e.g. #330) still
counts as a 'full build' per the earlier changed-files check, but the
cache step restores the same doc/_build/html as before since its key
is unchanged, and sphinx-gallery skips re-running any example script
that itself is unchanged. Net effect: CI can report success without
ever re-rendering the affected example.

Also there was no way to see a PR's doc build without checking it out
and building locally - docs.yml only publishes to GitHub Pages on
push to master. Upload the built HTML as a workflow artifact on every
run so reviewers can download and open it directly from the PR's
checks tab.

Author: pellet

.github/workflows/docs.yml

@@ -51,13 +51,26 @@ jobs:
       with:
         path: |
           doc/_build/html
-        key: ${{ runner.os }}-sphinx-${{ hashFiles('examples/**/*.py', 'doc/**/*', 'conf.py') }}
+        # Includes eegnb/**/*.py so a change to library code (not just an
+        # example or doc file) invalidates the cache too - otherwise the
+        # "full build" this triggers (see Determine build mode above) still
+        # restores a stale doc/_build/html, and sphinx-gallery skips
+        # re-running any example whose own script file is unchanged, so the
+        # rendered output never actually reflects the library change.
+        key: ${{ runner.os }}-sphinx-${{ hashFiles('examples/**/*.py', 'doc/**/*', 'conf.py', 'eegnb/**/*.py') }}
         restore-keys: |
           ${{ runner.os }}-sphinx-
 
     - name: Build docs
       run: make docs
 
+    - name: Upload built docs
+      uses: actions/upload-artifact@v4
+      with:
+        name: docs-build-preview
+        path: doc/_build/html
+        retention-days: 14
+
     - name: Deploy Docs
       uses: peaceiris/actions-gh-pages@v3
       if: github.ref == 'refs/heads/master'  # TODO: Deploy seperate develop-version of docs?