Re-freeze docs dependencies

[kurtmckee]

Read the Docs builds are still failing, but due to missing dependencies like Flask.

I created this PR with the following steps, iterating each time by trying to run make html locally.

# Set up an independent virtual environment
cd docs/
python -m venv .venv
source .venv/bin/activate

pip install sphinx
make html

# Needs Flask
pip install flask
make html

# Needs sphinxcontrib.httpdomain
pip install sphinxcontrib.httpdomain
make html

# Needs sphinx_rtd_theme
pip install sphinx_rtd_theme
make html

# Many warnings printed regarding RST syntax, directives, etc
# but there were no missing modules, so this was sufficient.
pip freeze > requirements.txt

As noted in the comment above, there are still many warnings about RST syntax and directives when building the docs, but at least the missing modules were accounted for. The .readthedocs.yaml file has fail_on_warning disabled so this should at least allow the builds to succeed.

Closes #347, too.