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.