Added an about page (#251)

This page was added since it is sometimes difficult to say which version
was deployed on an server. This should allow an easy lookup on the
server and make it comparable with what is expected.

tests/ui/about_test.py

@@ -0,0 +1,15 @@
+"""Tests if the about page can be created."""
+from dash.development.base_component import Component
+
+from aki_prj23_transparenzregister.ui import app
+from aki_prj23_transparenzregister.ui.pages import about
+
+_ = app
+
+
+def test_layout() -> None:
+    """Checks if the about page can be created."""
+    result = about.layout()
+    assert isinstance(result, list)
+    for e in result:
+        assert isinstance(e, Component)