[APM] Document OTel infrastructure tab support

[alexandra5000]

Summary

Updates the APM Infrastructure tab docs to reflect new behavior for OTel-instrumented services (available in 9.4):

• Previously, the Infrastructure tab only showed infrastructure observed by Metricbeat/Elastic Agent.
• Now, for OTel-instrumented services, the tab exclusively shows OTel-observed hosts, pods, and containers, with different click-through destinations (Hosts UI for hosts; Metrics in Discover for containers and pods).

Changes:

• solutions/observability/apm/infrastructure.md: Split into Elastic APM and OTel sections; added links to destination UIs.
• troubleshoot/observability/apm/common-problems.md: Added OTel-specific guidance to the "No data in infrastructure tab" troubleshooting section.

Relates to: elastic/kibana#252188
Closes: #5511

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: Claude Code (claude-sonnet-4-6)

[mdbirnstiehl]

Overall, LGTM! I think the applies_to needs to be updated from migration times, and one small suggestion you can take or leave!

[alexandra5000]

Hey @roshan-elastic, I made the update that you asked for in #5511. Could you review it please?

[roshan-elastic]

Hey @alexandra5000 - got a link?

[alexandra5000]

Hey @roshan-elastic, unfortunately the PR preview is temporarily disabled, so there's no way right now to generate a shareable link.

You can serve it locally:

1. Install docs-builder (if you haven't already):

curl -sL https://ela.st/docs-builder-install | sh

2. Check out my branch (apm-infrastructure-otel) in the docs-content repo.

3. Run from the repo root:

docs-builder serve

4. Open http://localhost:3000/

Full instructions are at contribute locally if you want more detail. Sorry for the inconvenience!

[roshan-elastic]

LGTM - thanks @alexandra5000 !