[8.x](backport #4269) [apm/otel] Add links to Elastic Distribution of OpenTelemetry PHP (#4272)

* add links to php otel distro (#4269)

(cherry picked from commit 94c19e4)

# Conflicts:
#	docs/en/serverless/apm-agents/apm-agents-opentelemetry.mdx

* Delete docs/en/serverless directory

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Co-authored-by: Colleen McGinnis <[email protected]>
Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>

Author: 3 people

docs/en/observability/apm/open-telemetry.asciidoc

@@ -51,6 +51,7 @@ Get started with an Elastic Distribution of OpenTelemetry language SDK:
 * preview:["The Elastic Distribution of OpenTelemetry .NET is not yet recommended for production use. Functionality may be changed or removed in future releases. Alpha releases are not subject to the support SLA of official GA features."] https://github.com/elastic/elastic-otel-dotnet[*Elastic Distribution of OpenTelemetry .NET →*]
 * preview:["The Elastic Distribution of OpenTelemetry Node.js is not yet recommended for production use. Functionality may be changed or removed in future releases. Alpha releases are not subject to the support SLA of official GA features."] https://github.com/elastic/elastic-otel-node[*Elastic Distribution of OpenTelemetry Node.js →*]
 * preview:["The Elastic Distribution of OpenTelemetry Python is not yet recommended for production use. Functionality may be changed or removed in future releases. Alpha releases are not subject to the support SLA of official GA features."] https://github.com/elastic/elastic-otel-python[*Elastic Distribution of OpenTelemetry Python →*]
+* preview:["The Elastic Distribution of OpenTelemetry PHP is not yet recommended for production use. Functionality may be changed or removed in future releases. Alpha releases are not subject to the support SLA of official GA features."] https://github.com/elastic/elastic-otel-php[*Elastic Distribution of OpenTelemetry PHP →*]
 
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