[otelcol] Rewrite the agent-to-gateway deployment page

[tiffany76]

• I have read and followed the Contributing docs, especially the "First-time contributing?" section.
• This PR has content that I did not fully write myself.
  • I used AI and I have read and followed the Generative AI Contribution Policy.
• I have the experience and knowledge necessary to understand, review, and validate all content in this PR.¹

This PR shuffles the Deploy section a bit, but mostly it rewrites the agent-to-gateway deployment pattern page. This pattern is popular with users and the current documentation lacked thorough explanations and examples.

EDIT: I asked an AI agent to analyze the #otel-collector Slack threads to find out what we really needed to add to this page. I tried vetting everything by cross-checking across different agents. And I spent a good chunk of time editing. Hopefully there aren't too many mistakes. 🤞

This is part of the cleanup from Phase 1 and contributes to #8793.

PREVIEW: https://deploy-preview-9173--opentelemetry.netlify.app/docs/collector/deploy/other/agent-to-gateway/

cc: @jade-guiton-dd and @mx-psi

Footnotes

1. Yes, I can answer maintainer questions about the content of this PR, without using AI. ↩

[tiffany76]

@jade-guiton-dd Any last thoughts before I hit merge?

[jade-guiton-dd]

Looks pretty good to me, just two minor concerns

[linux-foundation-easycla]

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[tiffany76]

Sorry for all the pings, folks! Git got the best of me. Since this is a complete mess, I'm closing now.

[tiffany76]

Some fairy godmother decided to work some git magic and now this PR is...back to where it should be? Tentatively going to try again.