ci: check rustdoc build on PRs

When we refactor rust crates, it's easy to break the rustdoc build by
forgetting to update pinned versions in our rustdoc build script:
`./deployments/scripts/rust-docs`. Let's run that script as part of the
Rust checks in each PR, so we get notified promptly about breakage.
We'll still only deploy new docs after PRs have been merged into main.

.github/workflows/rust.yml

@@ -51,6 +51,24 @@ jobs:
           # we must duplicate the rustflags from `.cargo/config.toml`.
           RUSTFLAGS: "-D warnings --cfg tokio_unstable"
 
+  docs:
+    # We use a custom script to generate the index page for https://rustdoc.penumbra.zone,
+    # and refactors to rust deps can break that generation. Let's ensure this script exits 0
+    # on PRs, but we'll still only deploy after merge into main.
+    name: Check that rustdocs build OK
+    runs-on: buildjet-16vcpu-ubuntu-2004
+    steps:
+      - uses: actions/checkout@v3
+        with:
+          lfs: true
+      - name: Install rust toolchain
+        # The script for rustdoc build requires nightly toolchain.
+        uses: dtolnay/rust-toolchain@nightly
+      - name: Load rust cache
+        uses: Swatinem/rust-cache@v2
+      - name: Build rustdocs
+        run: ./deployments/scripts/rust-docs
+
   wasm:
     name: Build WASM
     runs-on: buildjet-16vcpu-ubuntu-2004