Update Sphinx to fix build failures

One of Sphinx's default extensions now requires a newer version of Sphinx than what was used.

README.md

@@ -64,4 +64,4 @@ pip install -r docs/requirements.txt
 sphinx-build -b html docs build/docs/html
 ```
 
-Alternatively, you can use Docker to avoid changing your development environment, by running `docker run -it --rm -v ${PWD}/docs:/docs/docs -v ${PWD}/build:/docs/build -v ${PWD}/include:/docs/include sphinxdoc/sphinx:4.2.0 bash` to obtain a shell that you can use to run `apt-get update && apt-get install -y doxygen` and then the two commands above.
+Alternatively, you can use Docker to avoid changing your development environment, by running `docker run -it --rm -v ${PWD}/docs:/docs/docs -v ${PWD}/build:/docs/build -v ${PWD}/include:/docs/include sphinxdoc/sphinx:7.3.7 bash` to obtain a shell that you can use to run `apt-get update && apt-get install -y doxygen` and then the two commands above.

docs/conf.py

@@ -77,7 +77,7 @@
 #
 # This is also used if you do content translation via gettext catalogs.
 # Usually you set "language" from the command line for these cases.
-language = None
+language = 'en'
 
 # There are two options for replacing |today|: either, you set today to some
 # non-false value, then it is used:

docs/requirements.txt

@@ -1,3 +1,3 @@
-sphinx==4.2.0
-sphinx_rtd_theme==1.0.0
-breathe==4.31.0
+sphinx==7.3.7
+sphinx_rtd_theme==2.0.0
+breathe==4.35.0