docs: Add article "UCP Translates AP2 Requirements into Reality"

[mdoeseckle]

This commit adds an article discussing how UCP and AP2 protocols relate to one another.

[gemini-code-assist]

Summary of Changes

Hello @mdoeseckle, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces new documentation aimed at clarifying the relationship and compatibility between the Universal Checkout Protocol (UCP) and the Agent Payments Protocol (AP2). The added article details how UCP's features, such as its Checkout Object and various mandates, align with and fulfill the requirements of the AP2 specification, thereby providing a comprehensive guide for understanding their interoperability.

Highlights

• New Documentation Article: A new article titled "UCP Translates AP2 Requirements into Reality" has been added to the documentation.
• Protocol Compatibility Explanation: The new article clarifies how the Universal Checkout Protocol (UCP) is fully compatible with the Agent Payments Protocol (AP2), specifically through its AP2 Mandates Extension.
• Terminology Bridging: The documentation bridges the gap between UCP and AP2 terminology, explaining how UCP's 'Checkout Object' functions as the equivalent of AP2's 'CartMandate' and how 'CheckoutMandate' and 'PaymentMandate' fulfill AP2's user authorization requirements.
• Documentation Navigation Update: The main documentation index (docs/index.md) has been updated to include a link to this new article, ensuring it is easily discoverable.

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[gemini-code-assist]

Code Review

This pull request introduces a new documentation article explaining how the Universal Checkout Protocol (UCP) relates to the Agent Payments Protocol (AP2). The new content is clear and informative. My review includes a few minor suggestions to improve consistency, accuracy, and formatting in the documentation.

[gemini-code-assist]

For consistency with other documentation files (like core-concepts.md and specification.md), consider using **CartMandate** instead of **AP2 CartMandate**. The other documents refer to it simply as CartMandate.

Suggested change

	which serves as the functional equivalent of the **AP2 CartMandate**. Both
	which serves as the functional equivalent of the **CartMandate**. Both