write configuration docs for env var support

[david-luna]

Although the upstream NodeSDK and it subcomponents handle most of the configuration our Elastic Distribution handles some configurations which are necessary for GA. Also there is no specific document in upstream listing which env vars are supported and can be used to configure the agent.

The goal is not to have an extensive list of env vars with description and possible values since this is already in the spec. Instead we could point what is not supported yet since that list should shrink over time. The list in the document should contain the last update time