Archive OHW20 site as new release

[emiliom]

Archive ohw20 web site as https://oceanhackweek.github.io/ohw20/, to get ready to start updating the site for ohw21.

The archive/release approach set up in 2020 was fleshed out in #52.

When this is completed, edit the current site to have good pointers to ohw20 materials (and ohw19 & ohw18).

[emiliom]

I issued a ohw20 release just now. The system worked, and https://oceanhackweek.github.io/ohw20/ is now available. Though, it's worth noting that the associated jekyll-diff GH action had a failure

But maybe I should have created a projects page first, like https://oceanhackweek.github.io/ohw19/projects_2019.html? This page would be linked from a prominent button and the Menu as done in the ohw19 site. We can still do this by deleting the release then issuing it again when ready.

[abkfenris]

@emiliom '21 needs to be archived too.

[emiliom]

Moving the party to #109. Closing this issue