appdata: Rewrite the app's summary and description metadata

Rewrite the text to properly market the app,
while avoiding referencing the target environment (GNOME).

Broadly emphasize the UX, existing key features, intended use,
and benefits of the application for the user in their day-to-day lives,
without over-selling it either.

We can augment this description in the future when more core features land.

Fixes issue #1109

data/appdata/org.gnome.Calendar.appdata.xml.in.in

@@ -5,17 +5,26 @@
   <metadata_license>CC0-1.0</metadata_license>
   <project_license>GPL-3.0-or-later</project_license>
   <name>Calendar</name>
-  <summary>Calendar for GNOME</summary>
+  <summary>Manage your schedule</summary>
 
   <description>
     <p>
-      GNOME Calendar is a simple and beautiful calendar application designed to perfectly fit the GNOME
-      desktop. By reusing the components which the GNOME desktop is built on, Calendar nicely integrates
-      with the GNOME ecosystem.
+      GNOME Calendar is an elegant tool to help you plan and stay on top of your schedule for
+      upcoming days and weeks, whether your months are filled with meetings and appointments,
+      public events, travel logistics, or other commitments in your life.
     </p>
     <p>
-      We aim to find the perfect balance between nicely crafted features and user-centred usability. No
-      excess, nothing missing. You’ll feel comfortable using Calendar, like you’ve been using it for ages!
+      Easily create and manage unlimited calendars and events with GNOME Calendar's infinite
+      scrolling month view, week timetable, and scheduling list view.
+    </p>
+    <p>
+      Integrating the latest GNOME technologies and design best practices, GNOME Calendar builds on
+      top of the mature Evolution data server to manage large quantities of events and to provide
+      integration with other applications. You can add calendars from various online calendaring
+      services (like NextCloud, Google Calendar, and CalDAV / WebDAV servers), enabling you to
+      synchronize your events across multiple devices and platforms. GNOME Calendar also supports
+      local and offline calendaring, so you can use it anywhere and anytime while travelling
+      (or after civilization's collapse.)
     </p>
   </description>