Docs: Add documentation for setPaymentMethodAndPlaceOrder

[keharper]

Summary (*)

#723 added the setPaymentMethodAndPlaceOrder mutation. This mutation must be documented.

Examples (*)

Proposed solution

Add a topic in devdocs at guides/v2.3/graphql/reference/quote-set-payment-place-order.md

Note: Wait until #723 is merged into a specific branch to document this.

[keharper]

Fixed in magento/devdocs#4916