Improve documenation to work with serverless projects, API Key auth

[carsonip]

Serverless only accepts api key auth, not secret token auth.

1. elastic-helm deployment.yaml only takes secret token and passes it to otlp/elastic with hardcoded Bearer Authorization. See https://github.com/elastic/opentelemetry-demo/blob/main/kubernetes/elastic-helm/deployment.yaml#L74
2. docker compose flow is also broken because of https://github.com/elastic/opentelemetry-demo/blob/main/src/otelcollector/otelcol-elastic-config-extras.yaml#L7

[carsonip]

This is kind of mitigated in #90 but the meaning of ELASTIC_APM_SECRET_TOKEN becomes inconsistent with other usages.

[rogercoll]

The deployment will be simplified after #120, only an Elasticseach endpoint (YOUR_ELASTICSEARCH_ENDPOINT) and ApiKey (YOUR_ELASTICSEARCH_API_KEY) will be needed.