WIP: Update Sales/SOP for Z-billing and closing an opportunity clarity

Suggesting we update https://about.gitlab.com/handbook/sales/sop/#step-7--send-to-z-billing to state something similar to the following: Once a quote is signed, or a Purchase Order is received, attach it to the Zuora quote within the opportunity press the Send to Z-billing button.

Additionally, suggest we update https://about.gitlab.com/handbook/sales/sop/#step-8--close-won-an-opportunity changing "Once quote is signed and attached to opportunity, via Sertifi or manually and quote has been successfully sent to z-billing, go into the opportunity and change the stage to closed won." to... "Once quote is signed and attached to opportunity, via Sertifi or manually and quote has been successfully sent to z-billing, go back in to the opportunity and change the stage to Awaiting Approval followed by pressing the Submit for Approval button.