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'
```

.circleci/config.yml

@@ -148,7 +148,7 @@ workflows:
 jobs:
   quickcheck-docs:
     docker:
-      - image: cimg/base:2020.08
+      - image: cimg/base:2023.09
     steps:
       - checkout
       - install-docdep
@@ -279,7 +279,7 @@ jobs:
 
   publish-zrepl-github-io:
     docker:
-      - image: cimg/python:3.7
+      - image: cimg/base:2023.09
     steps:
       - checkout
       - invoke-lazy-sh: