docs: README refresh and docs update

[trentm]

Refs: #152

[david-luna]

here we are using the generic term oltp endpoint but next line we refer to elastic secret token which is more concrete to our stack. I wonder if we should add a note here telling APM server is an OTLP endpoint also

[trentm]

I'll change the example here to this:

---

# 1. install
npm install --save @elastic/opentelemetry-node

# 2. configure via OTEL_ envvars, for example:
export OTEL_EXPORTER_OTLP_ENDPOINT=https://{your-otlp-endpoint.example.com}
export OTEL_EXPORTER_OTLP_HEADERS="Authorization={authorization-information}"
export OTEL_SERVICE_NAME=my-service

# 3. start
node -r @elastic/opentelemetry-node my-service.js

If using an Elastic Observability deployment
to which to send telemetry data, the OTEL_EXPORTER_* settings will look
something like:

export OTEL_EXPORTER_OTLP_ENDPOINT=https://{deployment-name}.apm.{cloud-region}.cloud.es.io
export OTEL_EXPORTER_OTLP_HEADERS="Authorization=Bearer {deployment-secret-token}"