[DOCS] Add Entity analytics dashboard

[jmikell821]

Epic: https://github.com/elastic/security-team/issues/3062
PR: elastic/kibana#137688
Docs meta: #2477

Background

• New entity analytics dashboard is no longer hidden behind the experimental feature flag
• It requires the platinum license
• For now it only contains user and host risk scores.
• Add new doc to this section: https://www.elastic.co/guide/en/security/master/dashboards-overview.html

How to test it

• Enable entityAnalyticsDashboardEnabled, riskyHostsEnabled, and riskyUsersEnabled flags
• Go to the Entity Analytics page
• Click on "enable host risk score" button and follow the steps
• Click on "enable user risk score" button and follow the steps