[docs-infra] auto convert md images to next/image

Summary:
depends on D7207

this diff implements an imageTransformer which converts  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 [1]

we include it as part of the yarn snapshot script, together with the code transformer and lint format step.

Test Plan:
yarn snapshot

previews in https://github.com/dagster-io/dagster/pull/3944

Reviewers: sashank

Reviewed By: sashank

Differential Revision: https://dagster.phacility.com/D7138