Add docs related to the potential

[nstarman]

Transfer more docs from gala.

[adrn]

I didn't look in detail to see if it's a copy-paste, but if so, can you take along the git history from Gala? They are a bit out of date now, but I did spend a lot of time working on the docs fro Gala and having some transitive credit for that here would be nice :). But not if it's too much of a pain!

[nstarman]

I'll hunt down the git blame.

[nstarman]

@adrn, it would be much easier to sort out the docs + git history if you

• move the current docs and commit the removal
• add the full gala docs to galax
• re-add the current docs.

I can then go through the rest of the docs and comment out the not-yet-operable bits.
I don't think there's an easy way to transfer the git history otherwise.