feat(docs): add built-in help section with How-To guides embedded in app

- Add MkDocs Material docs build stage to Dockerfile and Dockerfile.local
- Mount pre-built docs as static files at /help/ in FastAPI (app/main.py)
- Add app/views/help.py with /help → /help/ permanent redirect route
- Register help router in app/views/__init__.py
- Add Help nav link to base.html (public + app nav, desktop + mobile)
- Create how-to guides: HP printer, ScanSnap, watched folder, email ingestion, mobile scanning
- Update mkdocs.yml with How-To Guides section and Material theme palette
- Add optional docs service (squidfunk/mkdocs-material) to docker-compose.yaml with docs profile
- Add mkdocs-material to requirements-dev.txt
- Add /docs_build to .gitignore
- Add tests for help view (8 tests, 100% coverage on help.py)

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

app/views/help.py

@@ -0,0 +1,27 @@
+"""
+Help documentation view routes.
+
+Serves the built MkDocs documentation site at /help.
+The static site is built during the Docker image build and placed at docs_build/.
+"""
+
+import logging
+import pathlib
+
+from fastapi import Request
+from fastapi.responses import RedirectResponse
+
+from app.views.base import APIRouter
+
+logger = logging.getLogger(__name__)
+
+router = APIRouter()
+
+# Path to the built MkDocs documentation
+_DOCS_BUILD_DIR = pathlib.Path(__file__).parents[2] / "docs_build"
+
+
+@router.get("/help", include_in_schema=False)
+async def help_redirect(request: Request) -> RedirectResponse:
+    """Redirect /help to /help/ so the MkDocs index is served correctly."""
+    return RedirectResponse(url="/help/", status_code=301)