added link over to full details on the procure to pay page

Why is this change being made?

I added a link to the https://about.gitlab.com/handbook/finance/accounting/#procure-to-pay from the https://about.gitlab.com/handbook/finance/procure-to-pay/#payment - the /payment page did not have full details on how vendors got paid.

NOTE: I did a search for the Google Drive named "Vendor Paid." as noted in the handbook and I saw no files there, so @cnunez maybe you want to review that? The current page also states that after that after payments are made and files are transferred to the google drive folder names "Vendor Paid." This marks the end of the Procure to Pay process. but based on the instructions here, https://about.gitlab.com/handbook/finance/accounting/#procure-to-pay, that seems a bit contradictory, at least from the eyes of a field marketer who's not in finance 😄

@cnunez once we chat through, I'll let you assign to Paul when we are ready!

Does this MR meet the acceptance criteria?

Conformity

• Added description to this MR explaining the reasons for the proposed change, per say-why-not-just-what