Docs: add documentation dependencies

[osheasienna]

Types of changes

• Documentation
• Maintenance / tooling (dependencies)

Motivation and context / Related issue

This PR adds/updates the dependencies required to build the documentation (Sphinx + theme/extensions).
It makes it possible to set up the docs environment via:
pip install -r docs/requirements.txt.

Related issue: N/A.

How has this been tested (if it applies)

Not run end-to-end locally (dependency list change only).
Intended verification:

• pip install -r docs/requirements.txt
• build the docs using the project’s documentation build.

PR checklist

• I have read the CONTRIBUTING document.
• The documentation is up-to-date with the changes I made (check build artifacts).
• All tests passed, and additional code has been covered with new tests.
• I have added the PR and Issue fix to the RELEASES.md file.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.87%. Comparing base (9f43898) to head (622a5ee).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #801   +/-   ##
=======================================
  Coverage   96.87%   96.87%           
=======================================
  Files         113      113           
  Lines       23062    23062           
=======================================
  Hits        22342    22342           
  Misses        720      720           

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