Centralizing documentation for standard columns

[gnguyen87]

There are documentations of models/columns that can repeat throughout all edu packages. We'd like a centralized .md documentation that reside in edu_wh that details the description of standard columns. These will be doc blocks that can be referenced in every downstream package in their .yml files.

This work includes the following steps:

• Find and write out standard descriptions for columns/models that appear frequently throughout all edu packages.
• Centralize them in a singular .md file that reside in edu_wh
• Refactor other packages .yml file to reference these doc blocks
• Test and run packages to ensure this all work