docs: switch to sphinx-multiversion for multi-versioned docs (#734)

The sphinxcontrib-versioning seems unmaintainted and I can't
get the fork that we used before this PR working on Python 3.10.

The situation wrt maintenance doesn't seem much better for
sphinx-multiversion, but, at least I could get it to work
with current sphinx versions.

The main problem with sphinx-multiversion is that it doesn't render
anything at `/`. I.e., `https://zrepl.github.io/configuration.html` will
be 404.
That's different from `sphinxcontrib-versioning`, and thus switching
to sphinx-multiversion would break URLs.
We host on GitHub pages and don't control the webserver,
so, we can't use webserver-level redirects to keep the URLs working.
We could create JS-level redirects, or `http-equiv`, but that's ugly as
well.
The simplest solution was to fork sphinx-multiversion and hard-code
zrepl's specific needs into that fork.
The fork is based off v0.2.4 and pinned via requirements.txt.
Here are its unique commits:
https://github.com/Holzhaus/sphinx-multiversion/compare/master...zrepl:sphinx-multiversion:zrepl

We should revisit `sphinx-polyversion` in the future once its docs
improve.
See
https://github.com/Holzhaus/sphinx-multiversion/issues/88#issuecomment-1606221194

This PR updates the various Python packages, as I couldn't get
sphinx-multiversion to work with the (very old) versions that were
pinned in `requirements.txt` prior to this PR.
This PR's `requirements.txt` is from a clean Python 3.10 venv on Ubuntu
22.10 after running

```
pip install sphinx sphinx-rtd-theme
pip install 'git+https://github.com/zrepl/sphinx-multiversion/@52c915d7ad898d9641ec48c8bbccb7d4f079db93#egg=sphinx_multiversion'
```

docs/publish.sh

@@ -26,8 +26,8 @@ checkout_repo_msg() {
     echo "clone ${GHPAGESREPO} to ${PUBLICDIR}:"
 }
 
-if ! type sphinx-versioning >/dev/null; then
-    echo "install sphinx-versioning and come back"
+if ! type sphinx-multiversion >/dev/null; then
+    echo "install sphinx-multiversion and come back"
     exit 1
 fi
 
@@ -49,11 +49,11 @@ else
     read -r
 fi
 
-pushd "$PUBLICDIR" 
+pushd "$PUBLICDIR"
 
 echo "verify we're in the GitHub pages repo..."
 git remote get-url origin | grep -E "^${GHPAGESREPO}\$"
-if [ "$?" -ne "0" ] ;then 
+if [ "$?" -ne "0" ] ;then
     checkout_repo_msg
     echo "finished checkout, please run again"
     exit 1
@@ -73,22 +73,15 @@ popd
 
 echo "building site"
 
-flags="$(python3 gen-sphinx-versioning-flags.py)"
-set -e
-sphinx-versioning build \
-   $flags \
-   docs ./public_git \
-   -- -c sphinxconf # older conf.py throw errors because they used
-                    # version = subprocess.show_output(["git", "describe"])
-                    # which fails when building with sphinxcontrib-versioning
-set +e
+python3 run-sphinx-multiversion.py . ./public_git
+
 
 CURRENT_COMMIT=$(git rev-parse HEAD)
 git status --porcelain
 if [[ "$(git status --porcelain)" != "" ]]; then
-    CURRENT_COMMIT="${CURRENT_COMMIT}(dirty)" 
+    CURRENT_COMMIT="${CURRENT_COMMIT}(dirty)"
 fi
-COMMIT_MSG="sphinx-versioning render from publish.sh - $(date -u) - ${CURRENT_COMMIT}"
+COMMIT_MSG="render from publish.sh - $(date -u) - ${CURRENT_COMMIT}"
 
 pushd "$PUBLICDIR"