Enhance Stripe billing portal documentation adding return_url option

[Mth0158]

Pull Request

Summary: I wanted to customize the return url used by Stripe checkout UI:

Related Issue: N/A

Description: The gem allows it, but it's not documented, hence my PR. https://github.com/pay-rails/pay/blob/33b8c2e9606dac1ff6c78c2e5bd6ce6f7b2a9b0f/app/models/pay/stripe/customer.rb#L226

Testing: N/A

Screenshots (if applicable): N/A

Checklist:

• [x] Code follows the project's coding standards
• [x] Tests have been added or updated to cover the changes
• [x] Documentation has been updated (if applicable)
• [x] All existing tests pass
• [x] Conforms to the contributing guidelines

Additional Notes: N/A