node: DRA: add docs for the DRAResourceSliceMixins FG

[ffromani]

Description

basic documentation of the feature gate

Issue

kubernetes/enhancements#5234

Closes: #N/A

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[ffromani]

/cc @mortent

to keep me honest with the docs here

[mortent]

We decided to punt this feature to 1.35 as described in kubernetes/enhancements#5234 (comment). So we can probably just close this PR.

[ffromani]

closing per #51475 (comment)