Fix doc build

[michaelkirk]

• I agree to follow the project's code of conduct.
• [n/a] I added an entry to CHANGES.md if knowledge of this change could be valuable to users.
• I ran cargo fmt

---

Fixes this error:

# This is pretty close to how docs.rs builds our docs
$ RUSTDOCFLAGS="--cfg docsrs" cargo +nightly doc --all-features       
 Documenting thiserror v1.0.69
 Documenting wkt v0.14.0 (/Users/mkirk/src/georust/wkt)
error[E0557]: feature has been removed
  --> src/lib.rs:19:29
   |
19 | #![cfg_attr(docsrs, feature(doc_auto_cfg))]
   |                             ^^^^^^^^^^^^ feature has been removed
   |
   = note: removed in 1.92.0; see <https://github.com/rust-lang/rust/pull/138907> for more information
   = note: merged into `doc_cfg`

error: Compilation failed, aborting rustdoc

I've also addressed some warnings while building docs and added a CI job to test the doc build.