docs: restore directory structure of the website

The `offline` plugin added to the MkDocs config in commit 772e2b8 ended up
changing the directory structure of the website, see the resulting commit
56aec196.

**Before** that commit, the following URLs were valid:

<https://martinvonz.github.io/jj/v0.9.0/revsets>
<https://martinvonz.github.io/jj/v0.9.0/revsets/>

and <https://martinvonz.github.io/jj/v0.9.0/revsets.html> was invalid.

The situation described above should be restored by this commit.

**After** 772e2b8 and before this commit, the following URLs were valid:

<https://martinvonz.github.io/jj/v0.9.0/revsets>
<https://martinvonz.github.io/jj/v0.9.0/revsets.html> and
<https://martinvonz.github.io/jj/v0.9.0/revsets/> was invalid.

This commit is likely to break the version switcher between v0.9.0 and newer
versions. The breakage would be minimized if this is merged shortly before a
release (but with enough time to test it out). Alternatively, I could try to
fix up the docs for that version.

We could also consciously decide to leave things as they are now.

The problem was noticed by @hooper, who noticed URLs in their browser history
that were no longer valid.

.github/workflows/release.yml

@@ -92,7 +92,7 @@ jobs:
       - name: Compile docs and zip them up
         run: |
           poetry install
-          poetry run -- mkdocs build --no-directory-urls
+          poetry run -- mkdocs build -f mkdocs-offline.yml
           archive="jj-${{ github.event.release.tag_name }}-docs-html.tar.gz"
           tar czf "$archive" -C "rendered-docs" .
           echo "ASSET=$archive" >> $GITHUB_ENV