BRP-17798 Improve UB docs

[mikelopez100]

BRP-17798

What changed?

• Updated the request URL shown to distinctly state that the developer should use their partner account UUID
• Add a much needed Possible Implementation Strategies to this page that should guide developers on how to integrate.
• Added some comments to the Create Checkout mutation to explicitly state which values should be used
• Added extra trials context to the Cancel Subscription mutation
• Added some common errors that are seen for Create Checkout

Release notes draft

• This will help those partners looking to utilize and integrate with our Unified Billing API

ping {names}

[chanchn]

looks good! consider my comment a 🍺 suggestion