[8.x](backport #4269) [apm/otel] Add links to Elastic Distribution of OpenTelemetry PHP

[mergify]

Add links to the Elastic Distribution of OpenTelemetry PHP in both stateful and serverless docs.

cc @estolfo @elastic/apm-agent-php

---

This is an automatic backport of pull request #4269 done by Mergify.

[mergify]

Cherry-pick of 94c19e4 has failed:

On branch mergify/bp/8.x/pr-4269
Your branch is up to date with 'origin/8.x'.

You are currently cherry-picking commit 94c19e41.
  (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/open-telemetry.asciidoc

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	deleted by us:   docs/en/serverless/apm-agents/apm-agents-opentelemetry.mdx

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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