Unify representation of events - part two - update other connector doc

For MySQL, there's a very detailed description of the change event fields in form of table: https://debezium.io/documentation/reference/connectors/mysql.html#_change_event_value. This should be done equivalently for the other connectors.

I updated the doc for MySQL in DBZ-2226

Also, when I did the Db2 doc, I updated its content for event descriptions based on my initial work for MySQL.

so the work for this Jira is to:

1. Update the Db2 content that describes change events with the latest doc for MySQL. 
2. Use the MySQL event descriptions to update the event descriptions doc for MongoDB, SQL Server, and PostgreSQL connectors.