support different versions

[ppanero]

Currently we only have "current" and "latest". We should be able to support "versioning" of those.

At the moment there are many issues open in mkdocs: https://github.com/mkdocs/mkdocs/issues/193, https://github.com/mkdocs/mkdocs/issues/614, etc.

There are two plugins available:

• https://github.com/d0ugal/mkdocs-versioned unsupported since 2016
• https://github.com/zayd62/mkdocs-versioning could work. Seems to be out of support too: https://github.com/zayd62/mkdocs-versioning/issues/40 but some kind souls offer to maintain it

I successfully built one version, however when trying the second one I run into https://github.com/zayd62/mkdocs-versioning/issues/32. Waiting for answers on it.