Consider hosting the doc on github pages

[jcohenadad]

Context

Hosting the doc on RTD has some issues:

• we've been struggling with RTD in the past (their own build environment is somewhat limited, and we've had some issues because of that)
• minor issue: RTD free version has some annoying ads (although it only costs $5/mo to get rid of it)
• minor issue: the URL is not super nice (i personally find "readthedocs.org" slightly condescending), whereas github.io looks better

It would be nice to find a solution for hosting a sphinx-built documentation outside of RTD. Recently i bumped into this great example: https://brainsprite.github.io/. This doc is hosted on github page, but built via sphinx by circle CI. The source is hosted there: https://github.com/brainsprite/brainsprite.github.io. I believe we could do something similar for this website: https://spine-generic.readthedocs.io/en/latest/, using github actions, triggered from this website: https://github.com/spine-generic/spine-generic. I've already created this "html hosting doc" repository here. So the next step would consist in setting up the GH action and deployment.

Some potentially useful sources:

• https://github.com/marketplace/actions/sphinx-build
• https://gist.github.com/c-bata/ed5e7b7f8015502ee5092a3e77937c99
• https://github.com/marten/github-sphinx-actions

[joshuacwnewton]

One (very minor) quirk to keep in mind for GitHub Pages:

https://brainsprite.github.io/ is using an "organization" GitHub Pages site and not a "project" GitHub Pages site, which gives it a shorter URL. This works because the organization has the the same name as the project repository (brainsprite/brainsprite), and that can also be done here for spine-generic/spine-generic.

For other projects (e.g. neuropoly/spinalcordtoolbox), the URL would have to be <organization>.github.io/<repository> instead. See Types of GitHub Pages sites for more info.