Set up reusable CI workflow and add building doctests

[ian-h-chamberlain]

Part of #106

This does a couple things:

• Makes sure we are actually building doctests now
• Switch to using https://github.com/actions-rust-lang/setup-rust-toolchain so we get nice inline warnings and caching
• Break out some common logic into a reusable composite action. I hope this can also be used for other crates going forward.

• TODO update the git hash once we merge Add limited support for cargo 3ds test --doc cargo-3ds#32

[Meziu]

This is great! Thanks for setting up the CI this way. The more automated checks we have, the easier it is to maintain the repo 👍

[Meziu]

I’ve made the suggestions to update this PR and merge it.