Clarify Language, value prop, audience is new-to-partnerships, differentiators added

Why is this change being made?

I want the audience for this page to be new (prospective) partners receiving a sales-like summary sales pitch on why to become a partner. Some of the language used is misleading or confusing, so I have clarified that. I want to make clear (and actionable) for a new partner the next steps in becoming a partner. I want to make clear the three main ways to make money with GitLab. I want to clarify GitLab’s differentiators from other Git-like tools in the marketplace. (Why sell GitLab over others?)

Author Checklist

• Provided a concise title for this Merge Request (MR)
• Added a description to this MR explaining the reasons for the proposed change, per say why, not just what
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