Expand on Express vs. Core

[jcheng5]

The goal with this PR is to end up with these articles under a "Syntax modes" section:

Filename	Title	Description
express-vs-core.qmd	Express vs. Core	Describes the motivation and details the differences from an authoring perspective
express-or-core.qmd	Choosing a syntax	Gives opinionated advice as to when you'd want to use one or the other
express-in-depth.qmd	Express in depth	Explores the fuzzy frontier where Express's abstractions start to get in the way, and the workarounds we've provided where possible
express-to-core.qmd	Transition to Core	Instructions for moving from Express to Core

[jcheng5]

I didn't intentionally make this express-[vs|or|to]-core.qmd, maybe revisit by end of the weekend