Add GitHub Actions docs workflow

.github/workflows/docs.yml

@@ -0,0 +1,75 @@
+name: Docs
+
+"on":
+  pull_request:
+  push:
+    branches:
+      - main
+  workflow_dispatch:
+
+permissions:
+  contents: read
+
+jobs:
+  validate:
+    name: Validate documentation
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v7
+
+      - name: Set up Python
+        uses: actions/setup-python@v6
+        with:
+          python-version: "3.12"
+          cache: pip
+          cache-dependency-path: requirements-docs.txt
+
+      - name: Check Dev Notes renderer syntax
+        run: python -m py_compile scripts/render-dev-notes.py
+
+      - name: Build documentation
+        run: scripts/build-docs.sh
+
+  deploy:
+    name: Deploy documentation
+    if: >-
+      github.ref == 'refs/heads/main' &&
+      (github.event_name == 'push' || github.event_name == 'workflow_dispatch')
+    needs: validate
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+      pages: write
+      id-token: write
+    environment:
+      name: github-pages
+      url: ${{ steps.deployment.outputs.page_url }}
+    concurrency:
+      group: github-pages
+      cancel-in-progress: false
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v7
+
+      - name: Set up Python
+        uses: actions/setup-python@v6
+        with:
+          python-version: "3.12"
+          cache: pip
+          cache-dependency-path: requirements-docs.txt
+
+      - name: Configure GitHub Pages
+        uses: actions/configure-pages@v6
+
+      - name: Build documentation
+        run: scripts/build-docs.sh
+
+      - name: Upload Pages artifact
+        uses: actions/upload-pages-artifact@v5
+        with:
+          path: site
+
+      - name: Deploy to GitHub Pages
+        id: deployment
+        uses: actions/deploy-pages@v5

README.md

@@ -16,7 +16,8 @@ OpenShell as the runtime.
   navigation entries from post front matter.
 - `zensical.toml` configures the documentation site.
 - `requirements-docs.txt` pins the documentation build toolchain.
-- `.gitlab-ci.yml` builds the documentation site in GitLab CI.
+- `.github/workflows/docs.yml` validates documentation builds in GitHub Actions
+  and deploys the generated site to GitHub Pages.
 
 ## Documentation workflow