Update install instructions, making them consistent for all sub-projects

[inteon]

Goal:
Reach base level of consistency for installation commands across all installation pages.
Once we reach consistency, we can think about improving these instructions (keeping in mind the CRD changes coming in 1.15).

Next steps:

1. Figure out when we should use helm upgrade --install vs helm upgrade vs helm install in our instructions & update.
2. Prepare 1.15 release (drastically simplify the CRD install section & use the single fool-proof install command everywhere)

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[wallrj]

1. Seems like unnecessary churn to me.
2. It introduces the cert-manager version everywhere, giving the false impression (I think) that istio-csr, csi-driver, etc depend on that particular version. As I already said in another review.
3. I think the goal should be to remove the helm **upgrade** instruction from each of the component install pages, because it risks users accidentally upgrading an important existing cert-manager installation and this change does not advance that goal.
4. I don't have time to test all the changes.

Have you tested everything?

[inteon]

Have you tested everything?
No, the commands have not changed, I only expanded the flags.
The only command that changed is the command that disables the approval-controller, and I copied that command from the approver-policy install page.

[cert-manager-prow]

PR needs rebase.

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