Publish documentation for the master branch of all Servo-maintained crates

[Ms2ger]

Right now, we have documentation on doc.servo.org for servo/servo master, and all dependencies, at whatever version master is using. This is useful, but if we want other people to use our crates, we should publish the most current documentation for them, even if we've not updated Servo to use the most current version.

We already do this for rust-url. It would be good to do that for all our crates.

Possible things to consider: https://github.com/huonw/travis-cargo; https://crates.fyi/crates/

[aneeshusa]

The way this is done in rust-url is uploading from Travis using an encrypted env var. See its .travis.yml for the dispatch and its Makefile for the actual doc creation + upload code.

travis-cargo seems to use the same flow but includes other things we don't need, so I'd like to avoid the extra dependency for now.

[nox]

Do we really need this, now we have docs.rs?

[nox]

Let's close it.