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_deploy.yaml

@@ -0,0 +1,95 @@
+# 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:
+    # Sync with github_actions_preview_fallback.yml on.pull_request.paths-ignore
+    paths:
+      - 'docs/**'
+      - 'github/workflows/docs**'
+      - '.hugo'
+
+jobs:
+  preview:
+    runs-on: ubuntu-24.04
+    defaults:
+      run:
+        working-directory: .hugo
+    concurrency:
+      # Shared concurrency group wih preview cleanup.
+      group: "preview-${{ github.event.number }}"
+      cancel-in-progress: true
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          fetch-depth: 0 # Fetch all history for .GitInfo and .Lastmod
+
+      - name: Setup Hugo
+        uses: peaceiris/actions-hugo@v3
+        with:
+          hugo-version: "latest"
+          extended: true
+
+      - name: Setup Node
+        uses: actions/setup-node@v4
+        with:
+          node-version: "22"
+
+      - name: Cache dependencies
+        uses: actions/cache@v4
+        with:
+          path: ~/.npm
+          key: ${{ runner.os }}-node-${{ hashFiles('**/package-lock.json') }}
+          restore-keys: |
+            ${{ runner.os }}-node-
+
+      - run: npm ci
+      - run: hugo --minify
+        env:
+          # HUGO_BASEURL: https://${{ github.repository_owner }}.github.io/${{ github.repository }}/previews/PR-${{ github.event.number }}/
+          # While private, GitHub uses an obfuscated url instead: 
+          HUGO_BASEURL: https://vigilant-guacamole-plnwrm9.pages.github.io/previews/PR-${{ github.event.number }}/
+          HUGO_ENVIRONMENT: preview
+          HUGO_RELATIVEURLS: false
+
+      - name: Deploy
+        # If run from a fork, GitHub write operations will fail.
+        if: ${{ !github.event.pull_request.head.repo.fork }}
+        uses: peaceiris/actions-gh-pages@v4
+        with:
+          github_token: ${{ secrets.GITHUB_TOKEN }}
+          publish_dir: .hugo/public
+          destination_dir: ./previews/PR-${{ github.event.number }}
+          commit_message: "stage: PR-${{ github.event.number }}: ${{ github.event.head_commit.message }}"
+
+      - name: Comment
+        # If run from a fork, GitHub write operations will fail.
+        if: ${{ !github.event.pull_request.head.repo.fork }}
+        uses: actions/github-script@v7
+        with:
+          script: |
+            github.rest.issues.createComment({
+              issue_number: context.payload.number,
+              owner: context.repo.owner,
+              repo: context.repo.repo,
+              body: "🔎 Preview at https://vigilant-guacamole-plnwrm9.pages.github.io/previews/PR-${{ github.event.number }}/"
+            })