Add Documentation

[RNViththagan]

Purpose

This PR enhances the ballerinax/paypal.subscriptions connector with comprehensive documentation and test instructions, providing clear setup, usage, and testing guidance for developers.

Changes

• Updated README.md with:
  • Overview: Describes the connector and PayPal Subscriptions API v1 support.
  • Setup Guide: Steps to set up a PayPal Developer account and obtain credentials.
  • Quickstart: Sample code to create a subscription plan.
  • Examples: Links to create-and-list-plans and monitor-and-manage-subscription examples.
• Added images (sandbox-accounts.png, create-account.png, create-app.png, get-credentials.png) to docs/setup/resources/ for the Setup Guide.
• Updated ballerina/tests/README.md with test setup, configuration, and instructions for running live_tests, mock_tests, and live_active_subscription_tests, linking to ballerina/tests/docs/ActivateSubscription.md for subscription activation.
• Added ballerina/tests/docs/ActivateSubscription.md with steps to manually activate a subscription for live_active_subscription_tests, including:
  • Creating a subscription with APPROVAL_PENDING status via testCreateSubscription.
  • Obtaining sandbox Personal account credentials.
  • Activating the subscription in the browser using the approve URL.
  • Updating Config.toml and running live_active_subscription_tests.
• Added test-related images (sandbox-accounts.png, personal-account-credentials.png, paypal-login-page.png, choose-payment-method.png, example-domain-page.png) to ballerina/tests/docs/resources/ for activation steps.

Checklist

• Updated the changelog

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