Add help page with user documentation

Create comprehensive help page covering:
- Getting started workflow
- Sites and IP management
- Scan configuration
- Running scans manually
- Scheduling automated scans
- Scan comparisons
- Alerts and alert rules
- Webhook configuration

Add Help link with icon to navigation bar.

app/web/templates/base.html

@@ -77,6 +77,12 @@
                     </li>
                 </ul>
                 <ul class="navbar-nav">
+                    <li class="nav-item">
+                        <a class="nav-link {% if request.endpoint == 'main.help' %}active{% endif %}"
+                           href="{{ url_for('main.help') }}">
+                            <i class="bi bi-question-circle"></i> Help
+                        </a>
+                    </li>
                     <li class="nav-item">
                         <a class="nav-link" href="{{ url_for('auth.logout') }}">Logout</a>
                     </li>