Document Tech Preview feature to collect Lambda extension logs

[simitt]

With #256 the lambda extension by default now has the Tech Preview feature enabled to collect logs from the lambda extension. The logs then show up in the Kibana APM UI.

The functionality is opt-out, users can disable it by setting ELASTIC_APM_LAMBDA_CAPTURE_LOGS=false.

When enabled, logs are collected and sent to the APM Server's new logs Intake API (also in Tech Preview) and indexed into Elasticsearch.

[lucabelluccini]

As this might create noisy logs if used against APM Server / Integration < 8.5, we should proceed documenting it if possible or switching to opt-in

Possible errors logged:

client error: response status code: 400
message: log: did not recognize object type

[simitt]

@bmorelli25 could you add this to your planning?