Pin sphinx-rtd-theme in requirements/docs.txt

After bumping Sphinx from 5.3.0 to 7.0.1, the docs build failed (see
https://readthedocs.org/projects/django-simple-history/builds/21210847/)
due to Read the Docs automatically installing an outdated version of
`sphinx-rtd-theme` (reason:
readthedocs/readthedocs.org#7858 (comment)).

Following the guidelines in their docs (see
https://docs.readthedocs.io/en/stable/guides/reproducible-builds.html#use-a-requirements-file-for-python-dependencies),
this commit pins the version of `sphinx-rtd-theme` - specifically to
the newest one.

requirements/docs.txt

@@ -1,3 +1,4 @@
 # Install this project in editable mode, so that its package metadata can be queried
 -e .
 Sphinx==7.0.1
+sphinx-rtd-theme==1.2.2