Add full documentation for Dynamic and Static Environments

[diemonster]

Our official docs need a full explanation of pros/cons of static versus dynamic environments. This discussion should at the minimum detail the following:

• Automatic scaling versus no scaling behavior
• Amazon ECR use for private repos using dynamic envs
• awsvpc networking mode as default for Dynamic envs and what that means
• caveats for edge cases in dynamic envs (i.e. you don't have direct access to the Docker bus)

[sesh-kebab]

We should also include what ever our recommendation (be it ECR credential helper or something else) for dealing with private repo authentication when using dynamic environments.

[tribaljack]

We're doing stateful vs stateless in task definition and at service level, not in environment level. closing as won't do.