Do not build docs for every version if there are not changes

[dgarcia360]

Context

The sphinx-multiversion extension builds docs for all the defined releases in conf.py.
Nevertheless, if an old version has not changed, generating docs for all releases is increasing the build time when some of the versions haven't received changes.

Implementation

The improvement is being discussed in sphinx-contrib/multiversion#63 and there is already a potential solution submitted sphinx-contrib/multiversion#64