[8.17](backport #4868) [otel] Use GitHub pages links

[mergify]

Description

Use GitHub pages links instead of links to Markdown files in the elastic/opentelemetry repo. A larger reworking of OpenTelemetry content in the Observability docs is happening in elastic/docs-content#985, but this fixes existing broken links in the pre-9.0 docs.

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

N/A

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>

---

This is an automatic backport of pull request #4868 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 2b8b1fa has failed:

On branch mergify/bp/8.17/pr-4868
Your branch is up to date with 'origin/8.17'.

You are currently cherry-picking commit 2b8b1fa4.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   docs/en/observability/apm/collect-application-data/open-telemetry/index.asciidoc
	modified:   docs/en/observability/quickstarts/monitor-k8s-otel.asciidoc

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   docs/en/observability/quickstarts/monitor-hosts-with-otel.asciidoc

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[github-actions]

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