asset catalog overview
This transforms the "Assets & Materializations" overview into an "Asset Catalog" overview. The goal is to distinguish between:
- Using Dagster to create / mutate assets, which is covered in the Asset Stores section.
- Recording that assets were created / mutated, which is covered in the Asset Catalog overview.
This will benefit heavily from screenshots, which I'm working on in a followup.
Test Plan: manual inspection
Reviewers: schrockn, yuhan, prha
Reviewed By: prha
Differential Revision: https://dagster.phacility.com/D5221