Add missing label for workflow:ready for design

Why is this change being made?

This MR adds missing information about the ~"workflow::ready for design" label to the Product Development Flow handbook page.

The optional label is currently being used to indicate an issue has been prioritized for product design because a problem is well understood and documented, and is ready for UX grooming of a solution. This MR adds a mention to the label, its description, and also expands the description of the ~"workflow::design" that was also missing from the page.

It also adds the product design manager as informed party in the design phase.

https://about.gitlab.com/handbook/product-development-flow/#validation-phase-3-design

Author Checklist

• Provided a concise title for the MR
• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
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  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to one of the people listed in the "Maintained by" section in on the page being edited.
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  • If this is a change that directly impacts the majority of global team members, it should be a candidate for #company-fyi. Please work with internal communications and check the handbook for examples.

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