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>

tests/test_views_help.py

@@ -0,0 +1,92 @@
+"""Tests for app/views/help.py module."""
+
+import pytest
+
+
+@pytest.mark.integration
+class TestHelpViews:
+    """Tests for the help documentation view routes."""
+
+    def test_help_redirect(self, client):
+        """GET /help should redirect permanently to /help/."""
+        response = client.get("/help", follow_redirects=False)
+        assert response.status_code == 301
+        assert response.headers["location"] in ("/help/", "http://testserver/help/")
+
+    def test_help_redirect_follows(self, client):
+        """Following /help redirect leads to /help/ (docs served or 404 if not built)."""
+        # In test environments docs_build/ is not present, so /help/ may 404.
+        # We only verify the initial redirect works; the final landing page depends
+        # on whether the docs have been built (they are built only in Docker images).
+        response = client.get("/help", follow_redirects=False)
+        assert response.status_code == 301
+        # Redirect target must be /help/
+        location = response.headers.get("location", "")
+        assert location.endswith("/help/")
+
+    def test_help_route_is_registered(self, client):
+        """Verify the /help route exists in the app router."""
+        # A GET to /help must not return 405 Method Not Allowed
+        response = client.get("/help", follow_redirects=False)
+        assert response.status_code != 405
+
+
+@pytest.mark.unit
+class TestHelpViewUnit:
+    """Unit tests for the help view module."""
+
+    def test_help_redirect_returns_301(self):
+        """Verify the redirect is HTTP 301 (permanent)."""
+        from fastapi import FastAPI
+        from fastapi.testclient import TestClient
+
+        from app.views.help import router
+
+        app = FastAPI()
+        app.include_router(router)
+        with TestClient(app) as tc:
+            resp = tc.get("/help", follow_redirects=False)
+        assert resp.status_code == 301
+
+    def test_help_redirect_target(self):
+        """Verify the redirect points to /help/."""
+        from fastapi import FastAPI
+        from fastapi.testclient import TestClient
+
+        from app.views.help import router
+
+        app = FastAPI()
+        app.include_router(router)
+        with TestClient(app) as tc:
+            resp = tc.get("/help", follow_redirects=False)
+        assert resp.headers["location"].rstrip("/").endswith("help") or resp.headers["location"].endswith("/help/")
+
+    def test_docs_build_path_constant(self):
+        """Verify _DOCS_BUILD_DIR is resolved relative to the app package."""
+        import pathlib
+
+        from app.views.help import _DOCS_BUILD_DIR
+
+        assert isinstance(_DOCS_BUILD_DIR, pathlib.Path)
+        # Should point to <repo_root>/docs_build
+        assert _DOCS_BUILD_DIR.name == "docs_build"
+
+
+@pytest.mark.integration
+class TestHelpNavigationLink:
+    """Tests that the Help link appears in the navigation."""
+
+    def test_help_link_in_nav(self, client):
+        """The Help navigation link should appear in the base template."""
+        response = client.get("/about")
+        assert response.status_code == 200
+        # The Help link should be present somewhere in the rendered page
+        assert b"/help/" in response.content
+
+    def test_help_link_has_accessible_text(self, client):
+        """The Help link should have visible text for accessibility."""
+        response = client.get("/about")
+        assert response.status_code == 200
+        content = response.text
+        # Should include the word "Help" associated with /help/
+        assert "Help" in content