Clarify required portions of the product development flow phases

Why is this change being made?

Related issue: #9090 (closed)

Introduce a pattern to clarify which portions of Product Development Flow phases are Required, by leveraging a required badge at the top of each phase section.

This MR does the following:

• Shows sample requirements for a some phases, which can be modified as the WG Contributors iterate
• Drops the required badge at the top of each phase section with 'TBD' as reminder to the WG Contributors to consider highlighting what, if anything, is required for each phase

The thought process is that if we clarify what is required in each phase, teams will have the latitude to adopt a working flow that best suites them, thereby making them more efficient.

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][transparency]
• Assign this change to the correct DRI
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  • If the changes relate to any part of the project other than updates to content and/or data files please make sure to ping @gl-static-site-editor in a comment for a review and merge. For example changes to .gitlab-ci.yml, JavaScript/CSS/Ruby code or the layout files.

Close no action

After discussion within the Product Development Flow Working Group, I will close this MR in favor of allowing the working group to divide and identify the primary outcomes(s). This exercise will naturally lead to determination of required vs. option phases, and allow for clarity in defining exactly what is required.