allowing contributors to re-trigger docs building

[edublancas]

@yafimvo shared in our session today that there is no way to re-trigger docs building

I suggested pushing an empty commit; the problem is that it'd trigger all the testing suites and in many cases, the author only wants to rebuild docs. we need to check if readthedocs allows this, perhaps it's just a permissions issue?

@yafimvo: are you looking to retrigger doc builds even when there are no code changes? what's the use case? re-trigger when docs fail to build because of some intermittent issue?