Move APM Guide docs to observability-docs repo

[bmorelli25]

For https://github.com/elastic/obs-docs-projects/issues/216

[github-actions]

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[bmorelli25]

@elasticmachine, run elasticsearch-ci/docs rebuild

[bmorelli25]

@elasticmachine, run elasticsearch-ci/docs rebuild

[bmorelli25]

@colleenmcginnis. Based on a new error, I've determined that the apm-server repository is a dependency of the Observability Guide in version 7.15. I did not test previous versions, but it's likely that this dependency exists in at least part or all of the versions between 7.15 and 7.9.

For this reason, I re-added the apm-server dependency for these versions in 943af47. After we get this PR building, we can probably copy the needed files to the observability-docs repository and remove this dependency again.

[bmorelli25]

@elasticmachine, run elasticsearch-ci/docs rebuild

[bmorelli25]

@elasticmachine, run elasticsearch-ci/docs rebuild

[colleenmcginnis]

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