[Security][Cases][9.3 & Serverless]: Search bar in case attachment tab

[nastasha-solomon]

Summary

Fixes #4330 by adding a tip to the case docs that explains what can be searched from the Attachments tab. The actual updates are linked below:

• Stack Management
• Observability
• Security

This PR also makes a few organizational and structural improvements to case docs. More specifically, it:

• Groups content about case attachments under a new section titled Add context and supporting materials. Previews below:
  • Stack Management
  • Observability
  • Security
• Moves duplicate content about adding alerts and observables into snippets. This should hopefully make future maintenance easier.

Generative AI disclosure

1. Did you use a generative AI (GenAI) tool to assist in creating this contribution?

• Yes
• No

[github-actions]

✅ Vale Linting Results

No issues found on modified lines!

[github-actions]

🔍 Preview links for changed docs

• explore-analyze/alerts-cases/cases/manage-cases.md
• solutions/observability/incident-management/create-manage-cases.md
• solutions/security/investigate/open-manage-cases.md

[mdbirnstiehl]

A few little things that you may or may not want to change! Otherwise, LGTM!