ci: Check for rustdoc warnings

[Techcable]

Right now, there are two rustdoc warnings that CI hasn't caught.

   Compiling slog v2.8.0-beta.2 (/Users/nicholas/git/slog.org/slog)
 Documenting slog v2.8.0-beta.2 (/Users/nicholas/git/slog.org/slog)
warning: this URL is not a hyperlink
    --> src/lib.rs:1332:30
     |
1332 |     /// default to `()` once https://github.com/rust-lang/rust/issues/29661 is
     |                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
     |
     = note: bare URLs are not automatically turned into clickable links
     = note: `#[warn(rustdoc::bare_urls)]` on by default
help: use an automatic link instead
     |
1332 |     /// default to `()` once <https://github.com/rust-lang/rust/issues/29661> is
     |                              +                                              +

warning: this URL is not a hyperlink
    --> src/lib.rs:1796:16
     |
1796 | /// Reference: https://github.com/rust-lang/rust/issues/34511
     |                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
     |
     = note: bare URLs are not automatically turned into clickable links
help: use an automatic link instead
     |
1796 | /// Reference: <https://github.com/rust-lang/rust/issues/34511>
     |                +                                              +

warning: `slog` (lib doc) generated 2 warnings (run `cargo fix --lib -p slog` to apply 2 suggestions)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 1.84s
     Opening /Users/nicholas/git/slog.org/slog/target/doc/slog/index.html

It would be good to catch these automatically.