Create a docs website

[php1ic]

Following on from #23, create a site on readthedocs.com and add an action workflow so it is automatically updated.

[php1ic]

Pipeline and a basic site have been set up - https://nuclearmasses.readthedocs.io/en/latest/index.html

It wasn't quite ready, but I had to merge #33 with main to publish the site. I'm therefore leaving this issue open while we add a bit of polish.