Document heterogeneous node groups for tasks

[Bihan]

Summary

• Add a Node groups section to the tasks concepts docs covering per-group nodes/resources/commands/ports and ${{ groups[i].nodes[j].IP_ADDRESS }} references.
• Split from the feature work in Support Heterogenous Node Groups #4094 so docs can be reviewed separately.

[peterschmidt85]

1. Do you think we could also add "???" (collapsible) section with P/D example?
2. Do you think it would make sense to also re-write our mpirun and ray examples - and link them from here?

[peterschmidt85]

IMO, this can use a blockquote to highlight this?

[Bihan]

@peterschmidt85

1. Do you think we could also add "???" (collapsible) section with P/D example?

P/D is a service setup, and the next PR switches services from replicas: to groups: and adds interpolation. I think it would be better to add it once that PR lands, when the example can use groups: and ${{ groups[i].replicas[j].IP_ADDRESS }}.

2. Do you think it would make sense to also re-write our mpirun and ray examples - and link them from here?
   Yes.

[peterschmidt85]

@Bihan

P/D is a service setup

I meant adding a task example