Agent Basic Usage - ECS docs

[iadjivon]

What does this PR do? What is the motivation?

This modifies the ECS docs to match the current UI.

This is a work in progress. These are the next action items:

• confirm and add the exact file the tile links to
• modify the content of said file per PM

Merge instructions

Merge readiness:

• Ready for merge

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Additional notes

[github-actions]

📝 Documentation Team Review Required

This pull request requires approval from the @DataDog/documentation team before it can be merged.

Please ensure your changes follow our documentation guidelines and wait for a team member to review and approve your changes.