Run cargo doc in CI

Signed-off-by: Alyssa Ross <hi@alyssa.is>

.buildkite/test_description.json

@@ -21,6 +21,10 @@
       "test_name": "style",
       "command": "cargo fmt --all -- --check --config format_code_in_doc_comments=true"
     },
+    {
+      "test_name": "doc",
+      "command": "RUSTDOCFLAGS=\"-D warnings\" cargo doc --workspace --all-features --no-deps"
+    },
     {
       "test_name": "unittests-gnu",
       "command": "cargo test --all-features --workspace",