Add case templates in serverless Observability docs

[lcawl]

This PR makes updates equivalent to #4049 and #4069 in this section of the serverless docs: https://www.elastic.co/docs/current/serverless/observability/cases

It adds some screenshots, renames a screenshot, and refreshes a screenshot that had an outdated navigation menu. Changes to the automated screenshots are accomplished via elastic/kibana#187815

Preview

• https://elastic-dot-co-docs-production-8v1z2nrqw-elastic-dev.vercel.app/current/serverless/observability/cases (screenshot refreshed)
• https://elastic-dot-co-docs-production-8v1z2nrqw-elastic-dev.vercel.app/current/serverless/observability/create-a-new-case (new step added for choosing template, "send cases..." section moved to this page)
• https://elastic-dot-co-docs-production-8v1z2nrqw-elastic-dev.vercel.app/current/serverless/observability/case-settings (page renamed and reorganized, automated images added, new template section added)

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[dedemorton]

Changes look great! Thanks!