Enable pdf and html downloads on readthedocs

[JimMadge]

✅ Checklist

• You have given your pull request a meaningful title (e.g. Enable foobar integration rather than 515 foobar).
• You are targeting the develop branch.
• Your branch is up-to-date with the develop branch (you probably started your branch from develop but it may have changed since then).
• If-and-only-if your changes are not yet ready to merge, you have marked this pull request as a draft pull request and added '[WIP]' to the title.
• If-and-only-if you have changed any Powershell code, you have run the code formatter. You can do this with ./tests/AutoFormat_Powershell.ps1 -TargetPath <path to file or directory>.

⤴️ Summary

• PDF and archived html docs are build and can be downloaded from readthedocs.io
• Remove exact version requirements from requirements.in
• Update requirements.txt

🌂 Related issues

🔬 Tests

[jemrobinson]

LGTM. I mildly prefer pinned requirements for reproducibility and ease of knowing when changes are made. Is there a reason this was removed or is requirements.in just the input for the final requirements.txt?

[JimMadge]

requirements question explained here: #1476 (comment)