Add CI support for doc tests. (#737)

As [`cargo-nextest` does not support running doc
tests](nextest-rs/nextest#16), we need a
`cargo test --doc` step as well.

.github/workflows/ci.yml

@@ -35,6 +35,10 @@ env:
 # Using cargo-hack also allows us to more easily test the feature matrix of our packages.
 # We use --each-feature & --optional-deps which will run a separate check for every feature.
 #
+# We use cargo-nextest, which has a faster concurrency model for running tests.
+# However cargo-nextest does not support running doc tests, so we also have a cargo test --doc step.
+# For more information see https://github.com/nextest-rs/nextest/issues/16
+#
 # The MSRV jobs run only cargo check because different clippy versions can disagree on goals and
 # running tests introduces dev dependencies which may require a higher MSRV than the bare package.
 #
@@ -217,7 +221,7 @@ jobs:
           sudo apt-get update
           sudo apt install -y xvfb libegl1-mesa libgl1-mesa-dri libxcb-xfixes0-dev mesa-vulkan-drivers
       
-      - name: Install cargo-nextest
+      - name: install cargo-nextest
         uses: taiki-e/install-action@v2
         with:
           tool: cargo-nextest
@@ -227,7 +231,7 @@ jobs:
         with:
           save-if: ${{ github.event_name != 'merge_group' }}
 
-      - name: cargo test
+      - name: cargo nextest
         # TODO: Maybe use --release; the CPU shaders are extremely slow when unoptimised
         run: cargo nextest run --workspace --locked --all-features --no-fail-fast
         env:
@@ -247,6 +251,9 @@ jobs:
             vello_tests/smoke_snapshots
             vello_tests/comparisons
 
+      - name: cargo test --doc
+        run: cargo test --doc --workspace --locked --all-features --no-fail-fast
+
   test-stable-wasm:
     name: cargo test (wasm32)
     runs-on: ubuntu-latest