Document schema for `gen-ca` command yaml file format

[ZackButcher]

Right now we use examples (eg AWS integration), but we don't actually show the fields that exist in the object for someone that wants to full it out. We should include documentation for those objects somewhere on the site - possible in the gen-ca reference docs?

The objects are spread across a few files:

• https://github.com/tetratelabs/getistio/blob/5d47165c6b2193653d9f37cd52d8de4c8a7342da/src/cacerts/providers/config/config.go#L41-L82
• https://github.com/tetratelabs/getistio/blob/5d47165c6b2193653d9f37cd52d8de4c8a7342da/src/cacerts/providers/models/models.go#L29-L84