added an about page

lib/pages/about_page.dart

@@ -0,0 +1,29 @@
+import 'package:flutter/material.dart';
+
+class AboutPage extends StatelessWidget {
+  const AboutPage({super.key});
+
+  static const String aboutText = '''
+This App was build for demonstration purposes.
+It requests data from the Environment Agency Real Time flood-monitoring API and displays a list and map of all flood measurement stations,
+as well as a graph showing the last 24 hours of measurements.
+The source code can be found at https://git.skup.in/.
+                          ''';
+
+  static const String routeName = '/about';
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      appBar: AppBar(
+        title: Text('About'),
+      ),
+      body: Center(
+        child: Text(
+          aboutText,
+          style: Theme.of(context).textTheme.bodyLarge,
+        ),
+      ),
+    );
+  }
+}