Add 'Switch to OpenTelemetry' migration guide

[alexandra5000]

Summary

Adds solutions/observability/get-started/opentelemetry/switch-to-otel.md, a new how-to guide for existing Elastic users migrating from classic APM agents or Beats-based collection to Elastic OpenTelemetry. Companion to #7875 (start-with-otel, for new users).

Closes #7321.

Generative AI disclosure

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

• Yes
• No

Tool(s) and model(s) used: Claude Code.

[github-actions]

Elastic Docs AI PR menu

Check the box to run an AI review for this pull request.

• Review docs changes (docs-review). Status: not started.

Powered by GitHub Agentic Workflows and docs-actions. For more information, reach out to the docs team.

[github-actions]

🔍 Preview links for changed docs

• solutions/observability/apm/metrics-ui.md
• solutions/observability/apm/opentelemetry/index.md
• solutions/observability/get-started/opentelemetry/start-with-otel.md
• solutions/observability/get-started/opentelemetry/switch-to-otel.md

[github-actions]

✅ Elastic Docs Style Checker (Vale)

No issues found on modified lines!

---

The Vale linter checks documentation changes against the Elastic Docs style guide. To use Vale locally or report issues, refer to Elastic style guide for Vale.