Auto-updating internal API docs

[hdevalence]

We don't publish the documentation anywhere because the crate isn't published, but for development it might be useful to set up some CI that builds the documentation with --document-private-items and pushes it to a server that we can link to. Firebase+Travis can certainly do this but I don't know if that's the best option here.

Steps:

• configure a domain alias (maybe under another subdomain, like zebra-internal.hackworks.zfnd.org) ?
• configure firebase for that domain
• add a CI step for publishing docs to firebase