Create documentation for installing and configuring the Agents in ECS

[jaffinito]

This will live in a shared agents location, not in .NET. it should be formatted to allow other agents to add collapsible sections with their specific instructions. Include links to sections in AWS docs for setting up actual cluster, no need to duplicate that - this would be in the shared section.

Should include:

• Required env vars
• An example app
• An example dockerfile for app and steps to use it
• An example task definition for using the example dockerfile
  • Using EC2
  • Using Fargate

[workato-integration]

https://new-relic.atlassian.net/browse/NR-291674

[jaffinito]

This is still onhold until #2658 is unblocked and completed.