ci: ensure docs are up to date

.github/workflows/docs.yaml

@@ -0,0 +1,37 @@
+# Copyright 2024 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+name: Check docs are up to date
+on:
+  pull_request:
+
+# Declare default permissions as read only.
+permissions: read-all
+
+jobs:
+  docs:
+    name: Check docs are up to date
+    runs-on: ubuntu-latest
+    steps:
+      - name: Setup Go
+        uses: actions/setup-go@41dfa10bad2bb2ae585af6ee5bb4d7d973ad74ed # v5.1.0
+        with:
+          go-version: "1.23"
+      - name: Checkout code
+        uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
+      - name: Generate docs and fail if there are differences
+        run: |
+          go install ./cmd/gendocs
+          gendocs > /dev/null
+          git diff --exit-code