Document how to self-host Editor UI

[timwis]

Per #204, it would be great if users could use the Editor UI even if their site isn't hosted on Netlify. I beileve it's possible by self-hosting gotrue and pointing Netlify CMS to it as the authentication provider. But getting gotrue up and running was more complicated than I anticipated (it seems like it does way more than we need, which is just GitHub OAuth).

I'd like to at least validate it's possible, and ideally document how to do it, for users who want/need to self-host, but still want the Editing UI.

EDIT: This repo may provide an option to hosting the authentication portion on vercel.

[timwis]

As mentioned in 204, this is much easier than I thought. We can add it to the documentation alongside how to run JKAN on GitHub Pages.