Document the deploy process

[eteq]

Since the switch back to gh-pages as a deploy mechanism in #283, we need to update the developer documentation. Hopefully we can just say it automatically uses circle-ci, but any gotchas revealed in #294 should be mentioned.

cc @adrn @Cadair

[kelle]

The deploy process has changed, but should still be documented. This issue could probably be transferred to https://github.com/astropy-learn/dev-guide/