Update autoscale docs

[jacobmalmberg]

What this PR does / why we need it:
Updates and clarifies docs on how to autoscale seldon deployments. Remove deprecated heapster as a prerequirement of seldon and autoscaling.

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[seldondev]

Hi @jacobmalmberg. Thanks for your PR.

I'm waiting for a SeldonIO or todo member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[seldondev]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[ukclivecox]

Thanks @jacobmalmberg