[DOCS] Create openAPI specifications for public APM APIs

[lcawl]

We need openAPI documents that are equivalent to these manually-maintained pages:

• APM server information API: [DOCS] Add openAPI specification for APM server information API #12862
• APM events intake API [DOCS] Add openAPI specification for APM events intake API #12982
• APM agent configuration API [DOCS] Add openAPI specification for APM agent configuration endpoints #12948
• openTelemetry intake API [DOCS] Add OpenTelemetry and Jaeger event intake API to OpenAPI document #13056
• jaeger event intake API [DOCS] Add OpenTelemetry and Jaeger event intake API to OpenAPI document #13056
• Serverless-specific subset of APIs: https://github.com/elastic/apm-managed-service/pull/788

Progress is currently blocked until #12904 is fixed.

Long-term, they'd ideally be generated automatically. In the near-term, they can be manually generated and maintained.