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>

Dockerfile.local

@@ -6,6 +6,19 @@ WORKDIR /app
 COPY requirements.txt /app/
 RUN pip install --no-cache-dir -r requirements.txt
 
+# ── Documentation build stage ───────────────────────────────────────────────
+FROM python:3.14.1-slim AS docs-builder
+
+WORKDIR /docs
+
+COPY docs/requirements.txt /docs/requirements.txt
+RUN pip install --no-cache-dir -r requirements.txt
+
+COPY docs /docs/docs
+COPY mkdocs.yml /docs/mkdocs.yml
+
+RUN mkdocs build --config-file /docs/mkdocs.yml --site-dir /docs/docs_build
+
 FROM python:3.14.1-slim
 
 WORKDIR /app
@@ -33,6 +46,9 @@ COPY ./LICENSE /app/LICENSE
 COPY ./VERSION /app/VERSION
 COPY ./BUILD_DATE /app/BUILD_DATE
 
+# Copy the pre-built MkDocs documentation site (served at /help)
+COPY --from=docs-builder /docs/docs_build /app/docs_build
+
 # Local fallbacks for build metadata
 RUN echo "local" > /app/GIT_SHA \
     && echo "local" > /app/RUNTIME_INFO