Documentation: Add note about executor for managed cluster apps

What does this MR do?

We received a support ticket in which a customer was having problems running the Managed-Cluster-Applications.gitlab-ci.yml template. It turned out they were trying to use a runner with the shell executor.

It's common that people will have small instances with only a couple of runners registered for proof-of-concept type projects. Since this is an alpha feature, this will likely be the type of environment where customers will test it out.

Looking at the template, I think it's simple to tell this requires a Docker-based runner (from the image) keyword, but I also think that adding a short note here removes the need for a debugging step at all. This may be somewhat of an edge case, but if it helps make for a smoother developer experience I think it's worth including.

Related issues

Here's the customer ticket: https://gitlab.zendesk.com/agent/tickets/162997 (internal only)

Author's checklist (required)

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• If you have developer access or higher (for example, GitLab team members or Core Team members)
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When applicable:

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