Document dark mode support for email layouts (#1397)

* docs: document dark mode support for email layouts

Explain that layouts created on or after April 21, 2026 support dark mode
by default, and show how customers with older layouts can opt in via the
prefers-color-scheme media query and the dark_logo_url / dark_icon_url
branding vars.

* docs: tighten email layouts for brevity

Cut redundant framing throughout the email layouts doc — reducing the
intro, default-layout tour, pre-content variables, visual-editor callout,
and CLI automation sections, and trimming the new dark mode section so it
doesn't duplicate what the code snippet already shows.

Author: samseely