407: Build Docs with Nightly r=kvark a=cwfitzgerald
This builds docs with nightly by default. We use intra-doc links which is currently only supported with nightly. docs.rs also uses the latest nightly to build our code, so this will match behavior with docs.rs. I added a fallback to stable in case nightly has issues so we still have working docs.
Co-authored-by: Connor Fitzgerald <connorwadefitzgerald@gmail.com>