publish latest rustdocs to gh-pages

[softprops]

Issue #, if available:

Description of changes:

Provide constantly updated docs by publishing rustdocs to ghpages on pushes to master. docs.rs will still serve its purpose as a release docs host. This will serve as the latest docs for anyone depending on the latest version in git. This might help clear up some confusion between releases.

This was adapted from a setup I use to automate processes in some of my other crates. An example can be found here

By submitting this pull request

• I confirm that my contribution is made under the terms of the Apache 2.0 license.
• I confirm that I've made a best effort attempt to update all relevant documentation.

[softprops]

This is interesting. cargo doc failed https://github.com/awslabs/aws-lambda-rust-runtime/runs/831196941?check_suite_focus=true

Finished dev [unoptimized + debuginfo] target(s) in 1m 07s
/home/runner/work/_temp/f51126fe-86e6-42f3-bdba-5b6a8b24ac43.sh: line 2: unexpected EOF while looking for matching ``'
##[error]Process completed with exit code 2.

Can you re-run that workflow run? There should be a control for that in the ui. I can't see it because I don't have permission

[davidbarsky]

Taking a look.