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>

frontend/templates/base.html

@@ -76,6 +76,12 @@
                {% if request and request.url.path == '/about' %}aria-current="page"{% endif %}>
               About
             </a>
+            <a href="/help/"
+               class="px-3 py-2 rounded-md text-sm font-medium text-gray-700 hover:text-gray-900 hover:bg-gray-100"
+               title="Help &amp; How-To Guides"
+               {% if request and request.url.path.startswith('/help') %}aria-current="page"{% endif %}>
+              <i class="fas fa-circle-question mr-1 text-gray-400" aria-hidden="true"></i>Help
+            </a>
 
             {% else %}
             {# ── App nav (logged-in or single-user / auth-disabled mode) ────── #}
@@ -171,6 +177,14 @@
               <i class="fas fa-circle-dot mr-0.5" aria-hidden="true"></i> Status
             </a>
 
+            <!-- Help documentation – visible to all users -->
+            <a href="/help/"
+               class="px-3 py-2 rounded-md text-sm font-medium text-gray-700 hover:text-gray-900 hover:bg-gray-100"
+               title="Help &amp; How-To Guides"
+               {% if request and request.url.path.startswith('/help') %}aria-current="page"{% endif %}>
+              <i class="fas fa-circle-question mr-1 text-gray-400" aria-hidden="true"></i>Help
+            </a>
+
             {% endif %}{# end multi_user_enabled / is_logged_in check #}
 
             <!-- Dark mode toggle -->
@@ -229,6 +243,11 @@
                  {% if request and request.url.path == '/about' %}aria-current="page"{% endif %}>
                 <i class="fas fa-info-circle mr-2 text-gray-400" aria-hidden="true"></i>About
               </a>
+              <a href="/help/"
+                 class="block px-3 py-3 rounded-md text-base font-medium text-gray-700 hover:text-gray-900 hover:bg-gray-50"
+                 {% if request and request.url.path.startswith('/help') %}aria-current="page"{% endif %}>
+                <i class="fas fa-circle-question mr-2 text-gray-400" aria-hidden="true"></i>Help
+              </a>
 
               {% else %}
               {# ── App links (logged-in or single-user) ────────────────────── #}
@@ -298,6 +317,13 @@
                 <i class="fas fa-circle-dot mr-1" aria-hidden="true"></i> Status
               </a>
 
+              <!-- Help documentation -->
+              <a href="/help/"
+                 class="block px-3 py-3 rounded-md text-base font-medium text-gray-700 hover:text-gray-900 hover:bg-gray-50"
+                 {% if request and request.url.path.startswith('/help') %}aria-current="page"{% endif %}>
+                <i class="fas fa-circle-question mr-2 text-gray-400" aria-hidden="true"></i>Help
+              </a>
+
               {% endif %}{# end multi_user_enabled / is_logged_in check #}
 
               <!-- Dark mode toggle (mobile) -->