docs(phase6): documentation site, landing page and open-source scaffolding

Material for MkDocs under docs/, covering self-hosting, the Rune, the
quick-add and filter languages, importing from Todoist, CalDAV,
mail-to-task, MCP, the AI layer and the API. The version is pinned: the
maintainers have announced that MkDocs 2.0 removes the plugin system
with no migration path, and an unpinned install would break this build
on the day it lands.

The landing page shows the quick-add parser as actual text rather than a
screenshot of it — that is the thing worth showing, and it reads better
as the real thing. It shares the app's design tokens, so the site and
what it describes look like one project.

MIT licence, contributing guide, security policy and issue templates.
Pages deployment is behind workflow_dispatch, so the docs build on every
push but nothing is published until the owner chooses to.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

.github/workflows/docs.yml

@@ -0,0 +1,58 @@
+name: Documentation
+
+# The docs are built on every push so a broken build is caught immediately, but
+# only published once GitHub Pages has been enabled for the repository. Until the
+# owner decides to publish, this job builds and stops.
+on:
+  push:
+    branches: [main]
+    paths:
+      - 'docs/**'
+      - 'mkdocs.yml'
+      - '.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:
+      - uses: actions/checkout@v4
+
+      - uses: actions/setup-python@v5
+        with:
+          python-version: '3.12'
+          cache: pip
+
+      # Pinned deliberately. The Material maintainers have announced that MkDocs
+      # 2.0 removes the plugin system and breaks every theme override with no
+      # migration path — an unpinned install would break this build on the day it
+      # lands, for a docs site nobody was editing.
+      - run: pip install 'mkdocs-material==9.7.7'
+
+      - run: mkdocs build --strict
+
+      - uses: actions/upload-pages-artifact@v3
+        with:
+          path: site_build
+
+  deploy:
+    needs: build
+    runs-on: ubuntu-latest
+    # Only runs once Pages is enabled. Until then the build above still guards
+    # against a broken docs tree.
+    if: github.event_name == 'workflow_dispatch'
+    environment:
+      name: github-pages
+      url: ${{ steps.deploy.outputs.page_url }}
+    steps:
+      - id: deploy
+        uses: actions/deploy-pages@v4