Change all documentation links to fees page

[aheckler]

In PAYOPS-1170, I updated and consolidated the fees documentation for WooPayments.

The new documentation lives here now: https://woocommerce.com/document/woopayments/fees/

Old fees docs URLs (such as https://woocommerce.com/document/woopayments/fees-and-debits/fees/) will redirect to the new page.

While the existing fees docs URLs in the plugin should work fine given this redirect, it's probably best to avoid the redirect where we can, so I'm filing this issue to update the URLs.