Is there a place to store ERDs in Postgres? Is there a designated spot in Postgres to store Entity-Relationship Diagrams (ERDs)? 

I could just create a table with a BTYEA column or something, but I want to make sure to use what exists if something already exists to document the database.
I recommend storing the XML of the ERD in a Code Repository (or Document Management System).  Read your Vendor's recommendation.

If you are building your own Document Manager, use a BLOB data type (which seems to be bytea in PostreSQL).