replace help.html with top-level index of Beaker docs

Also added a new item to the pill navigation, "develop", which is the
top-level index of the dev/ Sphinx docs.

This needs a corresponding commit to the Beaker docs, to make the
top-level index page match up with the former contents of help.html.

Change-Id: I13efab72a6cd80f0ea0f0704700f756c4b26f8b9
update submodules