Doc tweaks required to automatically build Db2 content in downstream user guide

I had to make some manual edits to content in the downstream user guide before I could build it. These edits are a result of the updates to the Db2 content, but the updates are not all in the db2.adoc file. Specifically: 

descriptions-of-debezium-postgresql-connector-data-change-events.adoc
In this section of the postgresql.adoc file, in the minitoc there is an xref that is missing "postgresql"

This downstream file has a double hyphen in the anchor ID:
assembly-deploying-debezium-db2-connectors.adoc

In db2.adoc, conditionalize this for community:

// Type: concept
// ModuleID:debezium-db2-connector-configuration-example
// Title: {prodname} db2 connector configuration example
[[db2-example]]
[[db2-example-configuration]]

Also there is a TBA example as the end of the db2.adoc file. Remove it.