Move juju HA into it's own topic

remove info about Juju HA from Charm HA page and make a new page for Juju HA.

The original HA page was too confusing because it tried to put too much into a single page.  Since Juju HA is really completely unrelated to Charm HA, I just split them into different pages.

The Juju HA docs have also been expanded quite a bit to make it much more clear how to set up HA and how to use it when things go wrong.