Closes #20961: Introduce RackGroup for physical rack placement (#21624)

Fixes #20961

docs/models/dcim/rackgroup.md

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+# Rack Groups
+
+Racks can optionally be assigned to rack groups to reflect their physical placement. Rack groups provide a secondary means of categorization alongside [locations](./location.md), which is particularly useful for datacenter operators who need to group racks by row, aisle, or similar physical arrangement while keeping them assigned to the same location, such as a cage or room. Rack groups are flat and do not form a hierarchy.
+
+Rack groups can also be used to scope [VLAN groups](../ipam/vlangroup.md), which can help model L2 domains spanning rows or pairs of racks.
+
+## Fields
+
+### Name
+
+A unique human-friendly name.
+
+### Slug
+
+A unique URL-friendly identifier. (This value can be used for filtering.)