[AAP-50873] docs: Add PostgreSQL extensions section with pg_stat_statements example

[arrestle]

Needed for [AAP-50873] Add database_resource_report management command

SUMMARY

This PR adds comprehensive documentation for configuring PostgreSQL extensions in AWX operator deployments. The new PostgreSQL Extensions section provides clear guidance on enabling extensions like pg_stat_statements for query performance monitoring.

The documentation includes practical examples showing both simple single-extension configuration and more complex multi-parameter setups. This addresses a common user need for enabling PostgreSQL extensions while maintaining the operator's flexible configuration approach.

ISSUE TYPE

• Bug, Docs Fix or other nominal change

ADDITIONAL INFORMATION

This is a documentation-only change that adds valuable guidance for users who need to enable PostgreSQL extensions. The examples demonstrate the proper use of postgres_extra_args to configure extensions at startup.

Key additions:

• Clear instructions for enabling pg_stat_statements extension
• Example combining multiple PostgreSQL configuration parameters
• Important notes about restart requirements and extension creation steps
• Consistent formatting with existing documentation style