Added information for non-finance team members for invoice payments

Why is this change being made?

As a non- finance team member, I've really struggled to understand how to deal with invoices. Pages have sent me to the "Procure to Pay" section to understand how invoices are paid, but the section seems to be written for the finance team.

I've added some formatting changes to help people scan the bullet points and understand what's happening in each. I've also added a little of text around that section to help anchor people who are landing there trying to figure out how to deal with invoices they've received from 3rd parties.

Author Checklist

• Provided a concise title for the MR
• Added a description to this MR explaining the reasons for the proposed change, per [say-why-not-just-what][transparency]
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