Add internal docs on how to update pooch files on repo

[IAlibay]

With #1814 there is an expectation for certain files to be updated whenever we want to add new test data from pooch.

However, this expectation wasn't communicated, nor was documentation provided anywhere.

[atravitz]

I'll add better instructions here.

Note that this doc s likely to change in the next ~week as we move away from Zenodo.