Link to Schema primitive types docs from Migration docs

[adamu]

The Ecto.Migration docs often mention types, and it's convenient to be able to cross-reference with the list of supported types. However, the Field Types section of the docs does not list the types, and in fact they're not listed anywhere in the package - instead being listed in the Ecto.Schema module of the ecto package.

This PR adds a link to the primitive types section of the Ecto.Schema docs from the Field Types section of the Ecto.Migration docs. It appears that ExDoc does not support dynamic linking to a section of a module doc, so I went with a full hyperlink to hexdocs.

Source:

Destination:

[greg-rychlewski]

Thank you!