Adds a git pull instruction to the documentation builder
If two consecutive commits happen in rapid succession, then the clone of the doc-builder repository might not be updated by the time the pip installations are over. This greatly reduces the risk this happens by putting a `git pull` instruction after the environment setup.
you could do it even later, i.e. 1/ building the doc to a temp folder, 2/ pulling, 3/ mv'ing the doc (instant), 4/ pushing