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?

[DragaDoncila]

@TimMonko so much great information here ❤️ A lot of it can be removed now that we're not using the .napari-hub/ folder and because we still are missing a lot of features on hub-lite 😭 I've left comments where appropriate.

[DragaDoncila]

Not sure how useful this table still is tbh. Everything comes from PyPI or manifest. Maybe it's worth editing down, but I'd also be happy to just remove it for now.

[DragaDoncila]

Can be removed.

[TimMonko]

Ok, oops, I just kinda went through all @DragaDoncila 's review and did one big massive commit. I think I have 1 or 2 things to address still, but thank you for the review and reminder to do this. Slipped my mind.

I need to look up the image thing for sure.

[TimMonko]

then what is it for? I was definitely interpolating or it was in the wiki docs
Are Categories used for anything?

[TimMonko]

Ok, now with instructions on how to get images and assets to render in READMEs :) This will certainly help me going forward, it's something I've wanted to figure out for a long time. This PR is greatly simplified now, about half as many LOC, so sorry that its a complete reorg. Should be an easier review now!

[DragaDoncila]

then what is it for? I was definitely interpolating or it was in the wiki docs
Are Categories used for anything?

Currently sadly, no. They need to be displayed on hub. I've added an issue on hub-lite (and opened a couple of others prompted by this PR), and I've triaged all the open issues using the critical and desired tags, to give us an idea of the work that still needs doing before we can consider the hub somehow "feature complete" - though of course, there are always more improvements to make.

[DragaDoncila]

@TimMonko two small comments and I think this will be ready to go!

[TimMonko]

Thanks for finishing this off @DragaDoncila , there have been enough Github notifs that I finally popped in but I guess you got it all ready to go. 🙇 🙏 ❤️

[DragaDoncila]

Ooops idk if I ever approved lol. This is ready to go afaic