Add documentation for Elastic Agent as an OTel Collector

[vishaangelova]

Summary

This PR adds a new “Elastic Agent as an OTel Collector” document with details about:

• Elastic Agent using OTel runtime for agent self-monitoring from v9.3
• Architectural changes and the Beats receivers as OTel receivers
• Elastic Agent as a hybrid agent and OTel integrations
• A comparison between the collector options available for ingesting OTel data
• FAQ

Resolves elastic/ingest-docs#1882

Generative AI disclosure

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

• Yes
• No

2. If you answered "Yes" to the previous question, please specify the tool(s) and model(s) used (e.g., Google Gemini, OpenAI ChatGPT-4, etc.).

Tool(s) and model(s) used: Cursor with claude-4.5-sonnet and Auto models

[github-actions]

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[github-actions]

🔍 Preview links for changed docs

• reference/fleet/elastic-agent-as-otel-collector.md

[theletterf]

Great work! First review pass.

[karenzone]

• I was originally thinking that this content belonged later in the TOC. Now that I see it, this seems like the right placement.
• The content hangs together nicely, and it's a good approach that will give users an idea of the "big picture without linking from place to place.
• I had similar/same comments wrt applies_to that you got from the previous reviewer.
• We should make a hard copyedit pass wrt tone and simplicity to make the content easy to parse and to align with existing docs. GitHub comments isn't the ideal tool for this kind of edits. Let's discuss other approaches.

[vishaangelova]

@nimarezainia Could you review this PR when your time allows it? Thank you!