docs: add resilience in Elastic Cloud Serverless page

[pete-naylor]

Summary

Adds a new page documenting resilience in Elastic Cloud Serverless, covering:

• Data durability — stateless storage architecture and object storage replication
• High availability — automatic zone distribution and SLA link
• Data recovery — Elastic-managed recovery data; notes that self-service recovery is planned
• Regional scope — resilience is regional; no built-in multi-region or cross-region replication

Also updates the parent "Design for resilience" page (availability-and-resilience.md) to include serverless: in applies_to and add a link to the new page, and adds the new page to toc.yml.

Changes

• deploy-manage/production-guidance/availability-and-resilience/resilience-in-serverless.md — new page
• deploy-manage/production-guidance/availability-and-resilience.md — add serverless: to applies_to; add link to new page
• deploy-manage/toc.yml — add new page after resilience-in-ech.md

Notes for reviewer

Follows the structure and conventions of resilience-in-ech.md. Avoids "snapshot/backup/restore" terminology for Serverless data recovery per product naming guidance — uses "recovery data" and "self-service data recovery" instead.

Preview

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🔍 Preview links for changed docs

• deploy-manage/production-guidance/availability-and-resilience.md
• deploy-manage/production-guidance/availability-and-resilience/resilience-in-serverless.md

[github-actions]

Elastic Docs Style Checker (Vale)

Summary: 1 suggestion found

💡 Suggestions (1): Optional style improvements. Apply when helpful.

File	Line	Rule	Message
deploy-manage/production-guidance/availability-and-resilience.md	69	Elastic.Wordiness	Consider using 'also' instead of 'In addition'.

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