[DOCS] Add openAPI specification for APM events intake API

[lcawl]

This PR creates an openAPI specification for documentation purposes to replace:

• https://www.elastic.co/guide/en/observability/current/apm-api-events.html
• https://www.elastic.co/guide/en/observability/current/apm-api-metadata.html
• https://www.elastic.co/guide/en/observability/current/apm-api-transaction.html
• https://www.elastic.co/guide/en/observability/current/apm-api-span.html
• https://www.elastic.co/guide/en/observability/current/apm-api-error.html
• https://www.elastic.co/guide/en/observability/current/apm-api-metricset.html
• https://www.elastic.co/guide/en/observability/current/apm-api-event-example.html

NOTE:

1. This PR re-adds the deleted OpenAPI document per outstanding Fix automation, bring back openAPI spec #12904.
2. The examples for error, span, and transaction events (copied from https://www.elastic.co/guide/en/observability/current/apm-api-event-example.html) are currently commented out because they fail to validate successfully against the specification. I'm assuming it's the examples that are at fault rather than the specification.
3. I've used redocly bundle command to import the referenced content into bundled.* files, since our API docs hosting site doesn't support the multi-file approach.

[mergify]

This pull request is now in conflicts. Could you fix it @lcawl? 🙏
To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

git fetch upstream
git checkout -b apm-events-intake upstream/apm-events-intake
git merge upstream/main
git push upstream apm-events-intake

[kruskall]

ugh, sorry about the openapi spec removal :(

I plan to figure out what's wrong with it soon