[Docs]: Nebius InfiniBand clusters (#2634)

Author: jvstme

docs/docs/concepts/fleets.md

@@ -71,7 +71,16 @@ This ensures all instances are provisioned in the same backend and region with o
     If the `aws` backend config has `public_ips: false` set, `dstack` enables the maximum number of interfaces supported by the instance.
     Otherwise, if instances have public IPs, only one EFA interface is enabled per instance due to AWS limitations.
 
-> The `cluster` placement is supported only for `aws`, `azure`, `gcp`, `oci`, and `vultr`
+??? info "Nebius"
+    `dstack` automatically creates an [InfiniBand cluster](https://docs.nebius.com/compute/clusters/gpu)
+    if all instances in the fleet support it.
+    Otherwise, instances are only connected by the default VPC subnet.
+
+    An InfiniBand fabric for the cluster is selected automatically.
+    If you prefer to use some specific fabrics, configure them in the
+    [backend settings](../reference/server/config.yml.md#nebius).
+
+> The `cluster` placement is supported only for `aws`, `azure`, `gcp`, `nebius`, `oci`, and `vultr`
 > backends.
 
 #### Resources

docs/docs/guides/troubleshooting.md

@@ -102,7 +102,7 @@ one of these features, `dstack` will only select offers from the backends that s
   are supported by all backends except `runpod`, `vastai`, and `kubernetes`.
 - [Clusters](../concepts/fleets.md#cloud-placement)
   and [distributed tasks](../concepts/tasks.md#distributed-tasks)
-  are only supported by the `aws`, `azure`, `gcp`, `oci`, and `vultr` backends,
+  are only supported by the `aws`, `azure`, `gcp`, `nebius`, `oci`, and `vultr` backends,
   as well as SSH fleets.
 - [Reservations](../reference/dstack.yml/fleet.md#reservation)
   are only supported by the `aws` backend.