Add support for keeping multiple versions of package manuals around

[fingolfin]

I.e. we could copy documentation into a subdir doc/VERSION/ instead of doc (or VERSION/doc), and let all doc/*.html files redirect to the latest version.

Of course then we'd also want a nice selector that shows all available versions of the manual and allows linking to older ones? Similar to https://docs.python.org/3/ or https://docs.julialang.org/en/v1/

See also gap-system/GapWWW#105

[fingolfin]

@ssiccha @danielrademacher this is what we talked about recently, seems I thought of this before 😂

This also requires work in https://github.com/gap-system/GitHubPagesForGAP