[ci] Publish cargo doc to GitHub Pages

[joshlf]

No description provided.

[joshlf]

Tested manually by setting the job to run on the pull_request trigger, which resulted in: https://google.github.io/zerocopy/

I've now removed that trigger.

[jswrenn]

Tested manually by setting the job to run on the pull_request trigger

Per-PR docs actually sounds awesome. Could you restore that functionality?

[joshlf]

Not without more work - the impl for testing just clobbered the docs every time a new PR is uploaded. It'd be cool to have separate docs for each PR, but it'd take more engineering work. We should do it as a follow-up though.