Add pricing themes addition for primary features

Why is this change being made?

Related to Product#2701 (comment 639082119)

This is an iteration to ensure we have a clear process to add pricing themes for new features as part of the release post. After we align and merge this MR, I will:

• further document and add references to the process as needed in the product handbook
• cross-link it as needed across the rest of the handbook
• add/clarify a process for changing existing themes
• create a pricing themes MR template for ease of use

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
  • Copy/paste the Slack conversation to document it for later, or upload screenshots. Verify that no confidential data is added.
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• If the changes affect team members, or warrant an announcement in another way, please consider posting an update in #whats-happening-at-gitlab linking to this MR.
  • 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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