Document the Schema.yml File

[jonwinton]

Is your feature request related to a problem? Please describe.
We need to document all the descriptive fields available on a schema.yml file in the core docs.

Describe the solution you'd like
A page should be made explicitly for the schema. From Kiln docs:

Schemas for components that are currently on the page are automatically parsed by Amphora and added to the page when Kiln loads. Schemas for other components are added when those components are added.```

Let's document each one of those fields and move other docs around upgrades and bookmarking into the core docs.