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

This should take the form of a "How-to" guide in [the diataxis sense](https://diataxis.fr/how-to-guides/).

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.