Pinning dependencies and adding contribution to the site

[ClemAtt]

This PR includes the following:

• Pins module versions where possible. I wasn't sure what to do in the case where you get something like this back from pip freeze @ file:///home/conda/feedstock_root/build_artifacts/mkdocs-material-extensions_1670252670575/work. This is now building OK though and actions run.
• Updated some of the versions for git actions modules in the actions .ymls e.g. actions/setup-python@v4 and actions/checkout@v3 as they were throwing deprecation warnings. Actions seem to run OK still.
• Added a last updated and contributors line to the PSD homepage. Wanted to use this for the contributors, but I think it only includes git page authors rather than repo contributors so have just manually added people that are named in PR and linked to git.

PS: just noticed there are data CSVs that I assume are updated as a result of the task?