Create managed_instances.md

[stzou]

What does this PR do? What is the motivation?

This PR adds instructions for setting up the Datadog agent with the new ECS Managed Instances launch type.

Merge instructions

Merge readiness:

• Ready for merge

For Datadog employees:

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Additional notes

[github-actions]

Preview links (active after the build_preview check completes)

New or renamed files

• https://docs-staging.datadoghq.com/stzou/ecs-managed-instances/containers/amazon_ecs/managed_instances

[cswatt]

First pass of edits!

[cswatt]

Pushed another round of edits to:

• remove unnecessary information (we don't need to sell ECS Managed Instances as a product; if it had a more opaque product name, we could theoretically explain what it is — but it's a very self-explanatory name)
• add some "how it works" type information
• consolidate repeated information
• put UI/CLI into tabs (and call them AWS Console and AWS CLI)
• other formatting changes