[Docs/RN] APM Package documentation is missing after 8.4.2 + document 8.15.0 changes in integrations page

[lucabelluccini]

The APM Integration package docs stopped being updated in 8.4.2 - see https://www.elastic.co/docs/current/integrations/apm#changelog

Full changelog at: https://github.com/elastic/integrations/blob/main/packages/apm/changelog.yml

I would also take the opportunity to mention the big changes introduced in 8.15.0 in the page https://www.elastic.co/docs/current/integrations/apm

Mention the removal from Fleet in favor of ES. References:

• Update docs to remove requirement for installing the integration #13237
• [apm] Update APM docs to reflect changes related to the Elasticsearch apm-data plugin observability-docs#4333

[lucabelluccini]

Additional question: how we will track the changelog/release notes of the APM assets in Elasticsearch? E.g. elastic/elasticsearch#112097 is not in the 8.15.1 release notes

[axw]

@lucabelluccini typically we do update the ES changelog, and I think we should have in that instance.