Add documentation for the columns in the Bulk Grade Download CSV

It seems that there is no documentation around the actual contents of the Bulk Grade Download CSV.
The feature is documented

here, in code
here, in the wild

but the contents are lacking any sort of explanation of the contents of the file.

Look at "18.3.2.1. Interpreting the Grade Report" and "18.3.6. Per-Learner Interventions Report", those each contain some amount fo breakdown of their respective files.

Add something like this for the Bulk Grades CSV Export

After updating, work with Support to validate the descriptive language. Likely, Jen Akana.

AC

• describe the data dictionary for the download file. Utilize language for end-users.

• Styling to match the surrounding sections.