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/routes/main.py

@@ -247,6 +247,18 @@ def alert_rules():
     )
 
 
+@bp.route('/help')
+@login_required
+def help():
+    """
+    Help page - explains how to use the application.
+
+    Returns:
+        Rendered help template
+    """
+    return render_template('help.html')
+
+
 @bp.route('/output/<path:filename>')
 @login_required
 def serve_output_file(filename):