move publishing from GitHub Pages to readthedocs

[tomkralidis]

To enable versioned documentation. This will allow tagged versions of documentation to be enabled.

• [x] setup on readthedocs.org (@tomkralidis)
• [ ] update training.wis2box.wis.wmo.int CNAME to point to readthedocs.io (note: this is the exact same setup as for docs.wis2box.wis.wmo.int) (@maaikelimper @kurt-hectic)
• [ ] add domain training.wis2box.wis.wmo.int to Read the Docs configuration (https://readthedocs.org/dashboard/wis2box-training/domains/), ensuring the domain is canonical/primary
• [ ] adjust table of contents (@tomkralidis)

[tomkralidis]

Setup on RTD completed. Once the CNAME is updated, then we will complete the domain setup in the RTD config, and finish the ToC update.