Propogate RuntimeSDK feature flag in docs

[killianmuldoon]

With #6383 a RuntimeSDK feature flag has been added to Cluster API. When the name and API has been agreed this feature flag should be documented in the Cluster API book and anywhere else it is required - similar to the CLUSTER_TOPOLOGY feature flag introduced with ClusterClass and managed topologies.

c/f #6383 (comment)

/area runtime-sdk
/kind cleanup

[killianmuldoon]

/assign

[sbueringer]

/milestone v1.2