[Documentation] Annotation to disable istio sidecar container

[andreyvelich]

/area docs

See comment: #1338 (comment).
While running Katib + Istio user should manually add sidecar.istio.io/inject: "false" annotation to Trial Template.
We should document it.

/cc @gaocegege @johnugeorge

/area release
/priority p1

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