Auto-generate documentation in main branch

[phihag]

Generate everything in the doc/ directory in the main branch, have a script to checkout gh-pages and do all that jazz.

Also, automatically update gh-pages: /documentation.html with the list of supported websites (encode it in youtube-dl and provide an option to read it).

[FiloSottile]

Probably this has become "Autogenerate gh-pages"

[jaimeMF]

I have added a script that generates the supported sites page in gh-pages: /supportedsites.html, the list is too long to be added to documentation.html.