Add Sphinx documentation and GitHub Pages deployment

Sphinx docs with Furo theme covering CLI usage, library API reference,
archive format specification, build instructions, history, and roadmap.
GitHub Actions workflow deploys to Pages on push to main.

.github/workflows/docs.yml

@@ -0,0 +1,45 @@
+name: Docs
+
+on:
+  push:
+    branches: [main]
+  workflow_dispatch:
+
+permissions:
+  contents: read
+  pages: write
+  id-token: write
+
+concurrency:
+  group: pages
+  cancel-in-progress: false
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: actions/setup-python@v5
+        with:
+          python-version: "3.12"
+
+      - name: Install Sphinx
+        run: pip install -r docs/requirements.txt
+
+      - name: Build docs
+        run: sphinx-build -b html docs docs/_build/html
+
+      - uses: actions/upload-pages-artifact@v3
+        with:
+          path: docs/_build/html
+
+  deploy:
+    needs: build
+    runs-on: ubuntu-latest
+    environment:
+      name: github-pages
+      url: ${{ steps.deployment.outputs.page_url }}
+    steps:
+      - id: deployment
+        uses: actions/deploy-pages@v4