Add special environemnts to build documentation (e.g. sphinx)

[RonnyPfannschmidt]

Hi,

in order to ease documentation handling and support the timely end if Make/bad,

i propose adding a way to have dcoumentation generation environment,s where the documentation format will be a factor

the basic idea is to have something like tox -e docs-html,docs-epub work nicely for any project choosing to use it

[gaborbernat]

I think our own tox.ini shows nicely how this can be done. The only thing to do here is to documented via an example, with sphinx/makedocs.