Edit onboarding flow to rewite image names in helm chart

[paigecalvert]

Updates the Replicated Onboarding flow to reflect the new proxy registry process (as in, telling users to edit image names in their helm values rather than using the HelmChart custom resource):

• Updates "Task 2: Rewrite Image Names in Helm Values to Use the Proxy Registry" to tell people to not only connect their image registry, but also rewrite image names in their Helm values.
  They’ll then add the SDK and package their chart before adding it to a new release.
• Updates "Task 4: Create the Initial Release with KOTS HelmChart and Embedded Cluster Config" to tell people to configure the values key in the HelmChart custom resource to inject the pull secret needed to authenticate with the proxy registry.
• Updates Add Support for Air Gap Installations to:
  • Add a step on how to configure optionalValues key to conditionally rewrite image names to the user's local image registry.
  • Remove some old Alpha notes about Helm air gap

Related to: #3209

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[ajp-io]

it's a little weird to do this later, because then you have to come back and amend these edits again. but that's okay if that's what makes sense.

[paigecalvert]

Ah that's a good point...For this pass, I'll probably just want to confirm that the adding custom domains task actually does correctly remind people to go back and edit these.

I'll want to give a little more time to considering if custom domains is perhaps something that should come earlier in the onboarding flow...onboarding is one of those things where it can be hard to pick a "step one" since there are several possible entry points