Clean up OpenTelemetry content

[colleenmcginnis]

Description

Removes OpenTelemetry content as described in #4843. This is the 8.x version of elastic/docs-content#985.

Documentation sets edited in this PR

Check all that apply.

• Stateful (docs/en/observability/*)
• Serverless (docs/en/serverless/*)
• Integrations Developer Guide (docs/en/integrations/*)
• None of the above

Related issue

Related to #4843, elastic/docs-content#985, elastic/docs-content#1099

Checklist

• Product/Engineering Review
• Writer Review

Follow-up tasks

Select one.

• This PR does not need to be ported to another doc set because:
  • The concepts in this PR only apply to one doc set (serverless or stateful)
  • The PR contains edits to both doc sets (serverless and stateful)
• This PR needs to be ported to another doc set:
  • Port to stateful docs: <link to PR or tracking issue>
  • Port to serverless docs: <link to PR or tracking issue>

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[colleenmcginnis]

As mentioned in elastic/docs-content#1099, I'm not sure what to do about Android and iOS. They are listed in the Elastic OpenTelemetry docs as EDOT SDKs, but those links go back to the APM agent docs where they are called Elastic OTel Android Agent and Elastic APM iOS Agent (not EDOT SDKs). Any guidance would be appreciated!

[theletterf]

Those distros currently lack documentation. I'd say let's link to the GH repos for now.

[theletterf]

Thanks @colleenmcginnis ! Added a comment on the iOS and Android distros, but otherwise LGTM.

[colleenmcginnis]

Closing. We can revisit if needed as a part of the discussion around #4843 (comment), but I think we'll want to keep this content for 8.x in the long term regardless of what happens with 9.x content.