Fix links in the code documentation

[VictorAvelar]

List of changes

• Update all the documentation links to point to Mollie's new documentation site.

Desired outcome

Currently the godoc block for creating a payment is:

// Create stores a new payment object attached to your Mollie account.
//
// See: https://docs.mollie.com/reference/v2/payments-api/create-payment

It should be:

// Create stores a new payment object attached to your Mollie account.
//
// See: https://docs.mollie.com/reference/create-payment