[docs-infra] auto convert md images to next/image
depends on D7207
this diff implements an imageTransformer which converts ![alt](path/to/img) to nextjs's Image component. details about next/image: https://nextjs.org/docs/api-reference/next/image
- note: next/image requires width and height so we use the imageTransformer to automatically inject the w/h based on the given image's size, see 
we include it as part of the yarn snapshot script, together with the code transformer and lint format step.
previews in https://github.com/dagster-io/dagster/pull/3944
Reviewed By: sashank
Differential Revision: https://dagster.phacility.com/D7138