Add case templates for serverless

[lcawl]

Relates to #5565

This PR makes similar changes as the aforementioned PR to restructure the case settings page and add the new template feature in the serverless documentation (in the https://www.elastic.co/docs/current/serverless/security/cases-overview section).

The newly automated screenshots are accomplished via elastic/kibana#187815

Preview

• https://elastic-dot-co-docs-production-h9cpiv9g1-elastic-dev.vercel.app/current/serverless/security/cases-open-manage
• https://elastic-dot-co-docs-production-h9cpiv9g1-elastic-dev.vercel.app/current/serverless/security/cases-settings

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[nastasha-solomon]

LGTM! Thanks for making these improvements and doc'ing the new functionality!