Elaborate on ecommerce in Why Astro

We've had several comments as to what we mean by

Astro was designed for building content-rich websites. This includes most marketing sites, publishing sites, documentation sites, blogs, portfolios, and some ecommerce sites.

(Discord message, support thread)

We could take the ecommerce mention out of that sentence and add a new sentence that describes in more detail Astro's suitability for ecommerce and some of the problems you might run into along the way.