[Doc] Document multi-agent _step TensorDict layout

[YeonwooSung]

Description

Extends docs/source/reference/envs_multiagent.rst with a Multiple agent groups section so a native EnvBase._step() is documented beyond the single "agents" group used by VMAS.

The new section:

• Shows the key tree a custom _step() must write for several groups ("red" / "blue", or "agents" / "adversaries"): per-group nested tensordicts with a stacked agent dimension.
• States where action (input only), observation, and reward live, and where done / terminated / truncated live when they are shared (root reset signal), per-group / per-agent (stacked), or one-group-per-agent.
• Requires specs to mirror that nesting, with a two-group spec example and an optional per-group done spec.
• Includes a copy-paste native TwoTeamEnv(EnvBase) sketch (not a PettingZoo wrapper) that reads (group, "action") and returns observations, rewards, and shared done flags without a "next" wrapper.
• Cross-links the competitive MADDPG tutorial as the canonical multi-group training loop.

The existing VMAS single-group example is kept as the simple case. The intro now treats "agents" as one group name, not a required key.

Motivation and Context

close #2425

The current multi-agent env page only documented the single "agents" group. Matteo Bettini agreed it needed a more general multi-group description; a later comment pointed at the competitive DDPG tutorial rollout as the pattern.

• I have raised an issue to propose this change (required for new features and bug fixes)

Types of changes

• Documentation (update in the documentation)

Checklist

• I have read the CONTRIBUTION guide (required)
• My change requires a change to the documentation.
• I have updated the tests accordingly (required for a bug fix or a new feature).
• I have updated the documentation accordingly.

[pytorch-bot]

🔗 Helpful Links

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❌ 3 New Failures

As of commit 853299d with merge base 1d3de3d ():

NEW FAILURES - The following jobs have failed:

• Libs Tests on Linux / unittests-gym (3.10, 12.8) / linux-job (gh)
  RuntimeError: Command docker exec -t 943dbf2815b431551359c8146c8220a5fe9101486e2f29ad6e970bd80198e356 /exec failed with exit code 1
• Unit-tests on Linux / tests-cpu (3.10, collectors) / linux-job (gh)
  test/test_collectors.py::TestCollectorGeneric::test_update_weights[MultiProcessWeightSyncScheme-False-False]
• Unit-tests on Linux / tests-cpu (3.14, bulk) / linux-job (gh)
  AssertionError: assert False

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