[DOCS][KSPM][8.4] Add info about Findings page to Elastic Security UI page 

## Description 

Add a brief section to the Elastic Security UI page describing the new KSPM Findings page and linking to the Kubernetes security posture management doc that explains how to set up the KSPM integration.

## Acceptance Test Criteria

Approved by Tinsae and docs team

## Notes

Meta issue: #2260 
