Keep documentation for all released versions

[mdbenito]

Update: See #8. Let's NOT use sphinx ~Update: There is something called sphinx-multiversion~

---

Manual approach:

• For each stable release, add a new folder vx.x.x in the branch gh-pages
• Base URL for all links within a release's docs should be properly set
• Create a root index.html with a redirect to the latest release

[MischaPanch]

Note that on readthedocs this is done automatically

[mdbenito]

Note that on readthedocs this is done automatically

Mike for mkdocs does this. Sphinx surely has some plugin too. I don't know why I gave some kind of lame steps...