Clarify URL-based payment method identifiers

[BHUVANSH855]

Description

This PR updates the URL-based payment method identifiers section of the Payment Request API concepts page by:

• Explaining how developers typically obtain URL-based payment method identifiers from payment service providers during onboarding or integration.
• Updating the Apple Pay and Google Pay examples with clearer browser support wording.
• Adding links to additional resources for developers interested in payment handler integrations.

Motivation

The current documentation only lists Apple Pay and Google Pay as examples without explaining how developers discover URL-based payment method identifiers.

Based on the discussion in the issue, this PR focuses on providing better guidance ("breadcrumbs") for developers instead of maintaining a list of payment providers. It also clarifies that support for non-built-in payment handlers is primarily a browser capability rather than a limitation imposed by a specific payment provider.

Additional details

The new "See also" resources point readers to official documentation for implementing payment handlers and integrating web payments, without expanding the documentation into a list of provider-specific implementations.

Related issues and pull requests

Fixes #38581

[github-actions]

Preview URLs (1 page)

• /en-US/docs/Web/API/Payment_Request_API/Concepts

External URLs (4)

URL: /en-US/docs/Web/API/Payment_Request_API/Concepts
Title: Payment processing concepts

• https://developer.samsung.com/internet/android/web-payments-integration-guide.html (1 time) (Note! This may be a new URL 👀)
• https://pay.google.com/payments/home/ (1 time) (Note! This may be a new URL 👀)
• https://web.dev/articles/android-payment-apps-developers-guide (1 time) (Note! This may be a new URL 👀)
• https://www.apple.com/apple-pay/ (1 time) (Note! This may be a new URL 👀)