IMprove MongoDB query performance.

src/Squidex.Domain.Apps.Read.MongoDb/Contents/MongoContentRepository_EventHandling.cs

@@ -62,6 +62,7 @@ namespace Squidex.Domain.Apps.Read.MongoDb.Contents
         {
             return ForAppIdAsync(@event.AppId.Id, async collection =>
             {
+                await collection.Indexes.CreateOneAsync(Index.Ascending(x => x.SchemaId).Descending(x => x.LastModified));
                 await collection.Indexes.CreateOneAsync(Index.Ascending(x => x.ReferencedIds));
                 await collection.Indexes.CreateOneAsync(Index.Ascending(x => x.IsPublished));
                 await collection.Indexes.CreateOneAsync(Index.Text(x => x.DataText));