Document pgaudit

[cristiklein]

We should create a sub-page to the Additional managed services postgresql page (or just an additional paragraph, not sure what's best) for the plugin pgaudit.

Add some general information about the plugin and how a managed service customer can ask for it to be installed, and what information that they need to provided - aka what databases.

Do mention that the way we will set it up is so that it's configured for Object-audit-logging. They will need to create the auditor role and give it permissions to the resources for which they want audit logs.

We should also mention and perhaps showcase that audit logs are visible in Opensearch.

[OlleLarsson]

Perhaps also add some disclaimer that using pgaduit might incur additional pressure of the postgres server and the logging stack.