Transition website away from Netlify and github pages

The Python 3.8 limitation with Netlify is untenable; among other things, 3.8 official support will end soon. Currently we use Netlify only for the website previews in PR's.

In addition, our pages are hosted on github pages using a scheme that allows us to host previously built pages (OHW19-21) together with newly built, Sphinx based pages. This, too, adds a burden.

See #249 and #237 for related, earlier discussions.

@abkfenris and I met a week ago to discuss this. The plan we came up with to move to ReadTheDocs for both website hosting and PR previews. It will entail:

• Test moving the ohw19-21 built pages from the master (github pages) branch to source. #314
• Move the ohw18 built pages from https://github.com/oceanhackweek/ohw2018 to source branch, like ohw19-21 pages.
• Simplify the environment build process, possibly using pixi
• Move to ReadTheDocs

We will not move the original Jekyll OHW18-21 files. We don't anticipate a need for editing those old files and rebuilding the pages with Jekyll. Future, occasional need for updating old pages (mainly fixing broken links, if at all) will be done directly on the built HTML pages.