[Feature] Document/update advice for atlas packaging developer workflow

[alessandrofelder]

Is your feature request related to a problem? Please describe.
As we launch a concerted effort to incorporate more atlases, we should streamline and standardise the process of adding atlases as much as possible

Describe the solution you'd like
We have some existing documentation for how to add an atlas (packaging script).
We should

• ensure it is up-to-date and clear, and update if needed
• add advice around pre-liminary steps (maybe as a checklist?). This should include
  • double-check the license
  • advice on how to evaluate the quality of the data visually
  • use the template script that we have, read the docs therein
• add advice on how to make the PR
  • include output of validation functions
  • include screenshots of the atlas in napari, including annotations, reference and at least one mesh