[don't merge!!] Add six dependency notice to building docs intructions

Don't merge this yet! Refs: #18113

ping @s0undt3ch @whiteinge 

This adds the dependency information about six if it is indeed needed to build the docs. I know @whiteinge is still investigating the errors. If the dep on six is needed, here's the doc change. If there is a better work-around discovered, feel free to just close this PR.