[ES3] Consolidate tech preview limitations + serverless differences, …

[leemthompo]

Addresses https://github.com/elastic/platform-docs-team/issues/537 + https://github.com/elastic/docs-content/issues/78#event-15284101032

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ℹ️ These changes focus on the ES3 docs, I don't know if we'll have time to consolidate and collaborate with other docs teams on moving this stuff up to a higher level before GA.

ℹ️ This is very drafty, if you'd prefer to work in the shadow Google Doc then comment there

Summary

Consolidate Elasticsearch tech preview limitations + serverless differences,clarify API/setting availabilities

• Better structural organization - Moving the serverless differences section earlier in the documentation hierarchy for visibility
• More detailed explanations of:
  • Infrastructure management changes
  • Specific API and settings availability
  • Clear categorization of features into replaced, planned, and permanently unavailable
  • Added clarity around error handling - The inclusion of specific error message examples helps developers understand and handle unavailable API responses.

🔨 TODO

• [x] Update alerting link

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