[Feature] How-to guide for creating a hierarchy csv

[alessandrofelder]

This should take the form of a "How-to" guide in the diataxis sense.

We should provide an example CSV file in brainglobe-template-builder for people to copy and edit, and (later?) some validation tooling.

It should include ~these steps:

• copy the csv file
• put each region in a row
• make sure ids match your annotation image
• detailed explanation of parent id tree column

Ensure good linkage and as little duplication as possible with the packaging new atlases guide.