Pin sphinx-rtd-theme to v1.0.0

It turns out that the newest version (1.2.2) requires Sphinx<7 (see
https://github.com/readthedocs/sphinx_rtd_theme/blob/1.2.2/setup.cfg#L48),
but version 1.0.0 doesn't (see
https://github.com/readthedocs/sphinx_rtd_theme/blob/1.0.0/setup.py#L121).
Either version seems to fix the build errors, so we could either
downgrade Sphinx to 6.2.1 or downgrade sphinx-rtd-theme to 1.0.0.
I ended up doing the latter, since an update to sphinx-rtd-theme seems
fairly close: readthedocs/sphinx_rtd_theme#1464.

requirements/docs.txt

@@ -1,4 +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
+sphinx-rtd-theme==1.0.0