Update source/handbook/finance/procurement/index.html.md

Why is this change being made?

Clarified Procurement Process by adding videos for the vendor contract approval issue creation process. Clarified legal and security role in process. To make processes easier to understand for someone who hasn't done it before.

cc @bryanwise

Does this MR meet the acceptance criteria?

Assign to DRI

• Did you assign this change to the correct DRI of the page or information you are changing?

Conformity

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