Elastic Security UI (#573)

- Cleans up the Solutions and use cases > Security > Get started >
Elastic Security UI page and consolidates serverless/stateful content.
- Removes serverless raw migrated file.
- Makes edits to reflect the solution view as the default UI.

Preview: [Elastic Security
UI](https://docs-v3-preview.elastic.dev/elastic/docs-content/pull/573/solutions/security/get-started/elastic-security-ui)

Author: natasha-moore-elastic

images/security-nav-overview.gif

@@ -290,7 +290,6 @@ toc:
       - file: docs-content/serverless/security-trusted-applications.md
       - file: docs-content/serverless/security-tune-detection-signals.md
       - file: docs-content/serverless/security-turn-on-risk-engine.md
-      - file: docs-content/serverless/security-ui.md
       - file: docs-content/serverless/security-view-alert-details.md
       - file: docs-content/serverless/security-visualize-alerts.md
       - file: docs-content/serverless/security-vuln-management-dashboard-dash.md