[ci] use cargo deadlinks to check for dead links

Since `cargo deadlinks` is just a link-checking wrapper around
`cargo doc`, we can simply replace our `doc` invocation with
`deadlinks` to get the benefits of both.

.github/workflows/ci.yml

@@ -270,12 +270,15 @@ jobs:
       # --document-hidden-items is unstable; if a future release breaks or removes it,
       # we can just update CI to no longer pass that flag.
       run: |
+        cargo install -q cargo-deadlinks --version 0.8.1
         # Include arguments passed during docs.rs deployments to make sure those
         # work properly.
         METADATA_DOCS_RS_RUSTDOC_ARGS="$(cargo metadata --format-version 1 | \
           jq -r ".packages[] | select(.name == \"zerocopy\").metadata.docs.rs.\"rustdoc-args\".[]" | tr '\n' ' ')"
+        # We invoke `cargo deadlinks` instead of `cargo doc`; `cargo deadlinks`
+        # is a link-checking wrapper around `cargo doc`.
         export RUSTDOCFLAGS="${{ matrix.toolchain == 'nightly' && '-Z unstable-options --document-hidden-items' || '' }} $RUSTDOCFLAGS $METADATA_DOCS_RS_RUSTDOC_ARGS"
-        ./cargo.sh +${{ matrix.toolchain }} doc --document-private-items --package ${{ matrix.crate }} ${{ matrix.features }}
+        ./cargo.sh +${{ matrix.toolchain }} deadlinks --check-intra-doc-links -- --document-private-items --package ${{ matrix.crate }} ${{ matrix.features }}
 
     # Check semver compatibility with the most recently-published version on
     # crates.io. We do this in the matrix rather than in its own job so that it
@@ -412,11 +415,12 @@ jobs:
           # in parallel.
           #
           # [1]  https://stackoverflow.com/a/42139535/836390
-          cargo check --workspace --tests            &> /dev/null &
-          cargo metadata                             &> /dev/null &
-          cargo install cargo-readme --version 3.2.0 &> /dev/null &
-          cargo install --locked kani-verifier       &> /dev/null &
-          cargo kani setup                           &> /dev/null &
+          cargo check --workspace --tests               &> /dev/null &
+          cargo metadata                                &> /dev/null &
+          cargo install cargo-readme --version 3.2.0    &> /dev/null &
+          cargo install cargo-deadlinks --version 0.8.1 &> /dev/null &
+          cargo install --locked kani-verifier          &> /dev/null &
+          cargo kani setup                              &> /dev/null &
 
           wait