Add Coordinator concept

[avrittrohwer]

What type of PR is this?

/kind documentation

What this PR does / why we need it:

Adds documentation for the Coordinator field.

Which issue(s) this PR fixes:

Fixes #701

Special notes for your reviewer:

Does this PR introduce a user-facing change?

NONE

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[danielvegamyhre]

/ok-to-test

[danielvegamyhre]

For completeness, can we include an example of the resulting label/annotation that would be applied to jobs/pods, for the example spec above? So the user can verify correctness and understand what they should expect the stable network endpoint to look like.

i.e., jobset.sigs.k8s.io/coordinator=pytorch-leader-0-0.pytorch

[avrittrohwer]

Added!

[danielvegamyhre]

/lgtm
/approve

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: avrittrohwer, danielvegamyhre

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