docs: add helm v3 commands

[masonlr]

What this PR does / why we need it:

This PR updates instances of helm install in the docs to be compatible with Helm v3 (the --name foo option for Helm release names is not supported in Helm v3 and the release name is now a positional argument).

This PR updates the docs, but doesn't update remaining Helm v2 statements in the following scripts/Makefiles:

• start-seldon-core-private-repo
• examples/cicd/cicd-argocd/seldon-core/start-seldon-core
• examples/cicd/sig-mlops-jenkins-classic/models/image_classifier/charts/seldon-model-server/Makefile
• examples/cicd/sig-mlops-seldon-jenkins-x/Makefile

@cliveseldon if you let me know I can update the scripts/Makefiles above to be compatible with Helm v3 as well – I'm not certain whether these are currently in use, particularly start-seldon-core-private-repo, so have left them alone.

Which issue(s) this PR fixes:

N/A

Special notes for your reviewer:

There is a separete PR that deals with similar changes to Istio content: #3044

For reference, the initial discussion was on Slack here: https://seldondev.slack.com/archives/CSVSD5S75/p1610047087283500?thread_ts=1609611010.277400&cid=CSVSD5S75

Does this PR introduce a user-facing change?:

NONE

[seldondev]

Hi @masonlr. 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.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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[seldondev]

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[axsaucedo]

This is great - thanks @masonlr !