Add docs for `nextstrain-pathogen.yaml` file

[joverlee521]

Follow up nextstrain/avian-flu#165 (comment)

Things to include:

• how it used by nextstrain build (related to Update "build directory" terminology in help and docs #371)
• how it is used to register compatibility with nextstrain run
• link to schema

Where to add:

• new page under config
• link to new page in nextstrain build, nextstrain setup, and nextstrain run

[victorlin]

Follow-up task: replace existing comment blocks with a link to this page (I will volunteer to do this as the suggestor of such a tedious task)