pip modules required to build docs not in requirements-dev.txt

[charliebudd]

Certain modules are missing from 'requirements-dev.txt' that are required to build the docs...
Sphinx
recommonmark
sphinx-autodoc-typehints
sphinx-rtd-theme

To Reproduce
Steps to reproduce the behavior:

1. Make a fresh pip/conda environment
2. Install requirementsfrom 'requirements-dev.txt'
3. Try to build docs (cd docs -> make html)
4. Fails due to missing modules

Expected behavior
'requirements-dev.txt' should contain all modules required for development

Additional Context
These modules are listed in the docs/requirements.txt but this is not immediately obvious to new developers