Test folding the OHW19-21 built files into `source`

[emiliom]

@abkfenris and I have discussed the current website infrastructure; see #237 and #249 for past, still-current discussions of the core issues. As a first step to simplifying the current setup and exploring a potential move to ReadTheDocs (#249), we're going to test folding into the current (starting with OHW22) site files the Jekyll-based built files from OHW19 to OHW21 (note: OHW18 files are in a separate repo, https://github.com/oceanhackweek/ohw2018). That'll be an incremental step towards deprecating the release/version-based hosting of github pages built files from OHW19-21.

After copying the OHW19-21 files, I'll test the Sphinx rendering locally and create a PR to test it on the Netlify preview.

Note: we use the master branch as the github pages branch.

[emiliom]

I've tested this. I copied the ohw19 folder from the master branch to my working branch, then built pages using make live.

Without any other changes, the only ohw19 page that was built was README.md! It looks like Sphinx is configured to ignore HTML and related files.

Doing some digging, I learnt about the html_extra_path configuration. When I set it to html_extra_path = ["ohw19"] (in conf.py), then ohw19 got built into the root folder, which isn't good (the files overwrote the root index, among other things). After some digging, I moved ohw19 to a subfolder, ohw18-21/ohw19 and set html_extra_path = ["ohw18-21"]. That did exactly what we want -- to create ohw19 files under a ohw19 folder. Except, the root index file was still overwritten with the ohw19 index page!

@abkfenris can you look into this? I may be stretching my Sphinx chops at this point.

Update: I've submitted a PR, #315, with these changes. Let's continue the discussion there.