Fix `cargo doc` warnings

[CosmicHorrorDev]

When running cargo doc I noticed some warnings about bare urls that aren't actually treated as hyperlinks. This PR just switches them to automatic links for ease of use when looking at the generated documentation.

I was hoping that I could add checking for doc warnings to CI, but it looks like there is still ongoing work for stabilizing that functionality. I don't want to switch to using unstable features that may change before stabilizing, but definitely good to keep an eye on for the future.