The sphinx .doctrees directory is not needed for publishing. By default
sphinx puts it below html. Some projects add to sphinx-build a "-d"
option to put the content elsewhere and not publish it.
This change assures that a .doctrees directory below "html" will not put
stored as artifact and published later.
Statistics: In all of OpenDev, we have:
* 511 repos putting .doctrees elsewhere and thus not publishing it
* 631 repos no caring and thus publishing .doctrees
The size of .doctrees directory is not neglectable: 2MB for
system-config, 14 MB for nova.
Change-Id: I63b581164aab84352d9e80278cf4a8d634ff28af
5eb4c257f5