node: DRA: add docs for the DRAResourceSliceMixins FG

content/en/docs/reference/command-line-tools-reference/feature-gates/DRAResourceSliceMixins.md

@@ -0,0 +1,19 @@
+---
+title: DRAResourceSliceMixins
+content_type: feature_gate
+_build:
+  list: never
+  render: false
+
+stages:
+  - stage: alpha
+    defaultValue: false
+    fromVersion: "1.34"
+---
+Enable support for Mixins in DRA ResourceSlice objects.
+There are scenarios where the number of devices can be pretty large and each device might have a relatively large amount of metadata associated with it,
+primarily in the form of Attributes or Capacity. The resulting objects can thus become very large.
+Mixins provides a way to define shared metadata for devices or counter sets separately from devices and then include them by reference.
+This enables less duplication of information in ResourceSlices and smaller footprint for each device in terms of size.
+
+This feature gate has no effect unless you also enable the `DynamicResourceAllocation` feature gate.