Docs: Update deps and enable reproducible builds

[CamStan]

Description

Add requirements.in file to list top-level docs dependencies.

Have rtd install pip-tools and use pip-compile to always generate an updated requirements file in the rtd build output.

Add the requirements.txt generated by pip-compile to pin this version of the docs as a reproducible build.

Note: When wanting to pin a new docs build, copy the results of pip-compile from the build logs and overwrite the existing requirements.txt

Additional changes:

• Update sphinx version and fix resulting conf.py build errors (language and extra css files).
• Add sphinx todo extension

Motivation and Context

Documentation builds started breaking as a result of ReadtheDocs changing what packages and versions are installed in the build environment. Using reproducible builds enables always having working builds of the docs in the event of future changes to the build environment.

https://blog.readthedocs.com/defaulting-latest-build-tools/

How Has This Been Tested?

Tested and built successfully from test-docs repository.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Testing (addition of new tests or update to current tests)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the UnifyFS code style requirements.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.
• All commit messages are properly formatted.