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

**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).](https://brainglobe.info/documentation/brainglobe-atlasapi/adding-a-new-atlas.html)
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