Add preinstalled software links to macOS image list

What does this MR do and why?

This MR adds links to the preinstalled software list for the macOS SaaS runner machine images to the list to make the resources easier to find.

@gabrielengel_gl what do you think about this change? Some feedback in Make it clear in the documentation that Flutter... (gitlab-org/incubation-engineering/mobile-devops/feedback#37) suggested that the preinstalled software list was difficult to find. I think this change would make it easier for folks to find what they're looking for.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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