Fix dependabot

[jemrobinson]

✅ Checklist

• You have given your pull request a meaningful title (e.g. Enable foobar integration rather than 515 foobar).
• You are targeting the appropriate branch. If you're not certain which one this is, it should be develop.
• Your branch is up-to-date with the target branch (it probably was when you started, but it may have changed since then).
• You have marked this pull request as a draft and added '[WIP]' to the title if needed (if you're not yet ready to merge).

⤴️ Summary

Dependabot is currently failing to correctly update requirements.txt from pyproject.toml (see e.g. #1809). It is also double-processing the requirements under docs.

This PR replaces the Python part of Dependabot's config with the following

• A new docs environment in pyproject.toml
• A workflow which uses pip-compile to update requirements.txt and docs/requirements.txt
• Changes all three update-X workflows to run both on pushes to develop and every week.

🌂 Related issues

Closes #1809

🔬 Tests

Untested

[JimMadge]

Looks good.

A few suggestions.
I would prefer not to run the update workflows periodically and on pushes.