Add KNN operator to the list of operators

[DelSkayn]

This PR adds documentation for the KNN operator.

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[DelSkayn]

@emmanuel-keller we don't seem to have a lot of documentation surrounding the knn operator.
Could you check if this added documentation is correct and maybe suggest a good example to show the use of the operator?

[emmanuel-keller]

@emmanuel-keller we don't seem to have a lot of documentation surrounding the knn operator. Could you check if this added documentation is correct and maybe suggest a good example to show the use of the operator?

Yes, documentation is coming soon. The feature is still experimental.