Add docs for adding a new core pathogen

[joverlee521]

Prompted by a lot of the recent work of adding new pathogens (e.g. seasonal-cov and rabies).

We should have a documented process for adding new core pathogens.

Steps to include:

1. Upload pathogen dataset to S3, i.e. nextstrain remote upload s3://nextstrain-data <local-path-to-jsons>
2. Add pathogen to coreBuildPaths for routing to work correctly.
3. Add pathogen to manifest_core.json to include pathogen in Auspice dataset dropdown and the /pathogens listing
4. Add pathogen to coreShowcase to include pathogen tile at the top of the /pathogens page.

[genehack]

5. Update description.md to link to S3 datasets?

[joverlee521]

5. Update description.md to link to S3 datasets?

I'm would think this would go into a more general doc of "How to ship a pathogen build". Here, I was thinking about specifically what needs to be done within nextstrain.org.