ci: GitHub Pages workflow for MkDocs site

Builds on push to main (docs/ or mkdocs.yml changes) and deploys to
https://pyr0ball.github.io/discarr via actions/deploy-pages.
Pinned to mkdocs-material 9.x (MIT, <10) — avoids 2.0 licence wall.

.github/workflows/docs.yml

@@ -0,0 +1,54 @@
+name: Deploy docs to GitHub Pages
+
+on:
+  push:
+    branches: [main]
+    paths:
+      - "docs/**"
+      - "mkdocs.yml"
+  workflow_dispatch:  # allow manual trigger from Actions UI
+
+# Grant GITHUB_TOKEN permission to deploy to Pages
+permissions:
+  contents: read
+  pages: write
+  id-token: write
+
+# Only one deployment at a time; cancel in-flight runs on new push
+concurrency:
+  group: pages
+  cancel-in-progress: true
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: actions/setup-python@v5
+        with:
+          python-version: "3.12"
+          cache: pip
+          cache-dependency-path: docs/requirements.txt
+
+      - name: Install MkDocs
+        run: pip install -r docs/requirements.txt
+
+      - name: Build site
+        run: mkdocs build --strict --site-dir _site
+
+      - name: Upload Pages artifact
+        uses: actions/upload-pages-artifact@v3
+        with:
+          path: _site
+
+  deploy:
+    needs: build
+    runs-on: ubuntu-latest
+    environment:
+      name: github-pages
+      url: ${{ steps.deployment.outputs.page_url }}
+    steps:
+      - name: Deploy to GitHub Pages
+        id: deployment
+        uses: actions/deploy-pages@v4