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/__init__.py

@@ -12,6 +12,7 @@ from app.views.filemanager import router as filemanager_router
 # Import all the view routers
 from app.views.general import router as general_router
 from app.views.google_drive import router as google_drive_router
+from app.views.help import router as help_router  # Built-in help / How-To docs
 from app.views.license_routes import router as license_router  # Add the license router
 from app.views.onboarding import router as onboarding_router
 from app.views.onedrive import router as onedrive_router
@@ -43,3 +44,4 @@ router.include_router(subscriptions_router)  # Pricing + subscription pages
 router.include_router(plans_router)  # Admin Plan Designer
 router.include_router(onboarding_router)  # User onboarding wizard
 router.include_router(pipelines_router)  # Processing pipelines
+router.include_router(help_router)  # Built-in help / How-To docs