docs: add Material for MkDocs documentation site (English + Japanese)

Add a documentation website under website/, built from README.md and
README-Japanese.md and laid out with top-tab topics and a left sidebar (same
style as the Hayabusa docs). Designed to be hosted free on GitHub Pages.

- Pages: Overview (About, Features, Screenshots), Getting Started, Commands
  (Command List, Command Usage), Resources (Companion Projects, Other
  Resources, Changelog, Contributing)
- Custom landing page, theme, click-to-zoom screenshots
- Changelog synced from CHANGELOG.md at build time
- 15-language switcher via mkdocs-static-i18n: English + Japanese full content;
  the other 13 localize the UI and fall back to English until translated
- .github/workflows/docs.yml builds (mkdocs --strict) + deploys to GitHub Pages

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.github/workflows/docs.yml

@@ -0,0 +1,66 @@
+name: Deploy docs to GitHub Pages
+
+# Build the Material for MkDocs site in website/ and publish it to GitHub Pages.
+# One-time setup: Settings -> Pages -> Source: "GitHub Actions".
+
+on:
+  push:
+    branches: [main]
+    paths:
+      - "website/**"
+      - "CHANGELOG.md"
+      - "CHANGELOG-Japanese.md"
+      - ".github/workflows/docs.yml"
+  workflow_dispatch:
+
+permissions:
+  contents: read
+  pages: write
+  id-token: write
+
+concurrency:
+  group: pages
+  cancel-in-progress: false
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v7
+        with:
+          fetch-depth: 0
+
+      - name: Set up Python
+        uses: actions/setup-python@v6
+        with:
+          python-version: "3.x"
+          cache: pip
+
+      - name: Install dependencies
+        run: pip install -r website/requirements.txt
+
+      - name: Sync changelog into docs
+        run: python website/scripts/sync_changelog.py
+
+      - name: Build site (strict)
+        run: mkdocs build --strict --config-file website/mkdocs.yml
+
+      - name: Configure Pages
+        uses: actions/configure-pages@v6
+
+      - name: Upload artifact
+        uses: actions/upload-pages-artifact@v5
+        with:
+          path: website/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@v5