Remove en/latest/ from readthedocs URL
Remove en/latest/ from readthedocs URL

After the doc is considered mature, we should switch the default version
to 'stable' instead of 'latest' (i.e. unreleased). Best is to advertize
the generic URL, and any change we do on the default version later will
be reflected on this URL.

hemberger/MechanicalSoup,hickford/MechanicalSoup,MechanicalSoup/MechanicalSoup