Subject: Fix race condition with sphinx and parallel builds
by using separate doctrees for the html docs and the manpage
This is in response to Debian bug https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=908756 and follows the fix found for ncmpc https://github.com/MusicPlayerDaemon/ncmpc/commit/d5aff222515ccd3ab8068796d5c13abf9329e11c