Migrate and update hub customization from wiki to docs

[TimMonko]

References and relevant issues

napari/napari-plugin-template#128

Description

1. Adds to docs and COMPLETELY REHAULS (so pls check carefully)https://github.com/chanzuckerberg/napari-hub/wiki/Customizing-your-plugin's-listing
   • uses pyproject instead of setup.cfg
   • uses newer project config metadata
   • clarifies differences in how metadata is acquired
   • doesn't really update the table (I had an LLM convert it), but I'm not really sure how important this is to have?
2. Also converts a smaller idea about descriptions from https://github.com/chanzuckerberg/napari-hub/wiki/Writing-the-Perfect-Description-for-your-Plugin
3. adds to Plugins -> Testing and publishing. Is this the appropriate place, or should it be in Building a Plugin?
4. Updates section in deploy to reference this new file

[TimMonko]

@jni since you requested this in napari/napari-plugin-template#128, pls review 😉 🙏
@DragaDoncila this is the conversion from the wiki we talked about, is there any other files that would be good here and an easy win? Or even just others I should think more about converting over in follow up PRs?