Document heterogeneous node groups for tasks

mkdocs/docs/concepts/tasks.md

@@ -179,6 +179,54 @@ Jobs on each node communicate using their private IP addresses. Use `DSTACK_MAST
     For convenience, `~/.ssh/config` is preconfigured with these options, so a simple `ssh <node_ip>` is enough.
     For a list of nodes IPs check the `DSTACK_NODES_IPS` environment variable.
 
+### Node groups
+
+A task can define multiple node groups. Each group has its own `nodes` count,
+`resources`, `commands`, and `ports`.
+
+<div editor-title=".dstack.yml"> 
+
+```yaml
+type: task
+name: ray-cluster
+
+python: 3.12
+
+groups:
+  - name: head
+    nodes: 1
+    commands:
+      - pip uninstall -y ray && pip install -U "ray[default]"
+      - ray start --head --port=6379 --block
+    resources:
+      cpu: 2
+      memory: 4GB..
+    ports:
+      - 8265
+
+  - name: workers
+    nodes: 2
+    commands:
+      - pip uninstall -y ray && pip install -U "ray[default]"
+      - ray start --address=${{ groups[0].nodes[0].IP_ADDRESS }}:6379 --block
+    resources:
+      gpu: H100:8
+```
+
+</div>
+
+Commands in any group can reference the internal IP address of any node in the run via
+`${{ groups[i].nodes[j].IP_ADDRESS }}`, where `i` is the index of the group in `groups` and `j` is
+the index of the node within that group.
+
+Node `groups[0].nodes[0]` is the run's master node — it is what `DSTACK_MASTER_NODE_IP` resolves
+to, and `startup_order` and `stop_criteria` apply to it across all groups.
+
+> Currently, only `resources`, `commands`, and `ports` can be configured per node group. [`groups`](../reference/dstack.yml/task.md#groups) and top-level `nodes` are mutually exclusive.
+> Group `resources` are not inherited from the task's top-level `resources`. A group that
+> omits `resources` gets the default values.
+> Support for other properties is coming soon.
+
 ### Resources
 
 When you specify a resource value like `cpu` or `memory`,