chore: Fargate docs update

[gabedos]

What does this PR do?

Update documentation to point root README to ECS Fargate docs.

Motivation

Users can easily be aware of ECS Fargate construct within the Datadog CDK Constructs repo.

Testing Guidelines

N/A

Additional Notes

Future work to be done:

Create a top level structure with the READMEs. Have the root highlight basic install info for the module and then direct to other more detailed READMEs for each different construct.

Types of Changes

• Bug fix
• New feature
• Breaking change
• Misc (docs, refactoring, dependency upgrade, etc.)

Check all that apply

• This PR's description is comprehensive
• This PR contains breaking changes that are documented in the description
• This PR introduces new APIs or parameters that are documented and unlikely to change in the foreseeable future
• This PR impacts documentation, and it has been updated (or a ticket has been logged)
• This PR's changes are covered by the automated tests
• This PR collects user input/sensitive content into Datadog