asset catalog overview

Summary:
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