Create a new page for Serverless cloud id

[yetanothertw]

Summary

Fixes #5150 and #345

The existing Find your Cloud ID page is ECH-only. Serverless projects also expose a Cloud ID (and an Elasticsearch endpoint / API keys), but the UI paths differ.

This PR:

• introduces a dedicated Serverless page
• adds a link to it on the existing ECH page

The new page includes the following info:

• covers the three connection concepts: Elasticsearch endpoint, API key, and Cloud ID (definition list + how-tos for finding the details).
• API keys: high-level entry points only, with a link to the existing Serverless API keys page (no duplicated procedure).
• Next steps point to Beats, Logstash, Elastic Agent, language clients, and the ingest overview (in the context of the purpose for which you'd want to connect your projects and tools/clients).

Generative AI disclosure

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

• Yes
• No

Cursor's auto agent to find similar or contradictory information within the existing docset.

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🔍 Preview links for changed docs

• deploy-manage/deploy/elastic-cloud/find-cloud-id.md
• deploy-manage/deploy/elastic-cloud/find-connection-details-serverless.md

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