Add sphinxext-rediraffe to docs/README.md

[bengtfrost]

Description

Added pip install sphinxext-rediraffe to docs/README.md file.

Checklist

Go over all the following points, and put an x in all the boxes that apply.

• [x] I have read the CONTRIBUTION guide (required)
• [x] I have linked this PR to an issue using the Development section on the right sidebar or by adding Fixes #issue-number in the PR description (required)
• [x] I have checked if any dependencies need to be added or updated in pyproject.toml and uv lock
• [x] I have updated the tests accordingly (required for a bug fix or a new feature)
• [x] I have updated the documentation if needed:
• [x] I have added examples if this is a new feature

If you are unsure about any of these, don't hesitate to ask. We are here to help!

[review-notebook-app[bot]]

Check out this pull request on 

See visual diffs & provide feedback on Jupyter Notebooks.

---

Powered by ReviewNB