dev/sg: render 'sg ci preview' with nice markdown

Created by: bobheadxi

gen-pipeline.go currently only renders very bare-bones output since it is a separate tool. Perhaps we could roll it into sg proper one day, but for now it seems the simplest way to get nicer output is to render Markdown and use the pretty Markdown printer introduced in https://github.com/sourcegraph/sourcegraph/pull/30532 to get some formatted results.

Also supports some emoji through glamour's built-in emoji renderer option