Add Sphinx documentation with Diataxis structure and GitHub Pages deployment

Addresses community feedback (gforcada) requesting contributor docs for
adding migration rules and a maintenance strategy overview.

- 21 pages: tutorials, how-to guides, reference, explanation
- MyST Markdown, shibuya theme, MXMAKE build system
- GitHub Actions workflow for auto-deploy to Pages
- README slimmed down, links to full docs

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: jensens

.github/workflows/docs.yaml

@@ -0,0 +1,58 @@
+---
+name: Documentation
+
+on:
+  push:
+    branches: [main]
+    paths:
+      - "docs/**"
+      - ".github/workflows/docs.yaml"
+  workflow_dispatch:
+
+permissions:
+  contents: read
+  pages: write
+  id-token: write
+
+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: "3.13"
+
+      - uses: astral-sh/setup-uv@v4
+
+      - name: Install docs dependencies
+        working-directory: docs
+        run: |
+          uv venv .venv
+          uv pip install sphinx sphinx-autobuild \
+            myst_parser sphinxcontrib.mermaid shibuya \
+            sphinx-design sphinx-copybutton
+
+      - name: Build documentation
+        working-directory: docs
+        run: .venv/bin/sphinx-build sources html
+
+      - uses: actions/upload-pages-artifact@v3
+        with:
+          path: docs/html
+
+  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