[DOCS] Add links in APM APIs

[lcawl]

This PR adds links from https://www.elastic.co/guide/en/observability/master/apm-annotation-api.html and https://www.elastic.co/guide/en/observability/master/apm-agent-key-api.html to https://www.elastic.co/docs/api/doc/kibana/group/endpoint-apm-agent-keys and https://www.elastic.co/docs/api/doc/kibana/group/endpoint-apm-annotations.
The latter pages are derived from https://github.com/elastic/kibana/blob/main/x-pack/plugins/observability_solution/apm/docs/openapi/apm.yaml

The pages in the Observability Guide can be deleted when the generated docs are satisfactory.

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