docs: add site for hosting docs on github pages (#204)

Adds a static site generation using Hugo, and deployed to GitHub pages
(googleapis.github.io/genai-toolbox).

.github/workflows/docs_preview_clean.yaml

@@ -0,0 +1,59 @@
+# Copyright 2025 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: "docs"
+
+permissions:
+  contents: write
+  pull-requests: write
+
+# This Workflow depends on 'github.event.number',
+# not compatible with branch or manual triggers.
+on:
+  pull_request:
+    types:
+      - closed
+
+jobs:
+  clean:
+    if: ${{ !github.event.pull_request.head.repo.fork }}
+    runs-on: ubuntu-24.04
+    concurrency:
+      # Shared concurrency group wih preview staging.
+      group: "preview-${{ github.event.number }}"
+      cancel-in-progress: true
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          ref: gh-pages
+
+      - name: Remove Preview
+        run: |
+          rm -Rf ./previews/PR-${{ github.event.number }}
+          git config user.name 'github-actions[bot]'
+          git config user.email 'github-actions[bot]@users.noreply.github.com'
+          git add -u previews/PR-${{ github.event.number }}
+          git commit --message "cleanup: previews/PR-${{ github.event.number }}"
+          git push
+
+      - name: Comment
+        uses: actions/github-script@v7
+        with:
+          script: |
+            github.rest.issues.createComment({
+              issue_number: context.payload.number,
+              owner: context.repo.owner,
+              repo: context.repo.repo,
+              body: "🧨 Preview deployments removed."
+            })