Add Marketing specific monthly variance analysis to FP&A handbook

Why is this change being made?

This MR describes the marketing specific monthly process used by the FBP to prepare budget line item level BvsA analysis using campaign tagging applying to Marketing programs spend, allowing us to join expense data from a variety of sources (Netsuite, Expensify, GitLab Finance Issues, Departmental Budgets) and roll them up at the budget line item level for campaigns, events, etc...

The business purpose of this process is to provide accurate and detailed information to accounting by BD4 during the close process i.e. for accruals, and to the marketing team for spend analysis vs internal departmental budgets. From a finance perspective, this will provide a valuable source for BvsA color commentary when comparing to plan/fcst.

We successfully stepped through this process for the January (FY20) close, allowing us to provide accounting with required information by BD4. This is related to Q1 OKR found here

cc @cmcclure1 @kstithem @darawarde @daleb04 @wwright

Does this MR meet the acceptance criteria?

Yes

Conformity

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