Update WooPay global theme settings copy

[pierorocca]

Description

We will update the new global theme support label and copy to better express the value to the merchant, to include a link to documentation, and to be consistent with case, spacing, and font used in settings.

Acceptance criteria

1. The title is changed to "Checkout theme"
2. The checkbox label is changed to "Enable global theme support"
3. Add secondary text, on the next row: "When enabled, WooPay checkout will be themed with your store’s brand colors and fonts. Learn more [opens in new tab icon]."
4. Font family, size, and relative placement matches this pattern:
   

cc @hsingyuc

[pierorocca]

@aheckler could we get a placeholder url for this documentation that you could attach here please?

[aheckler]

It'll almost certainly go in the checkout appearance section:

https://woocommerce.com/document/woopay-merchant-documentation/#checkout-appearance