Restructure the README around the core points

[JeroenDeDauw]

Follows-up to #6

_{AI-authored — Claude Code, Fable 5 (max); text-review ask from @JeroenDeDauw, redirected twice (lead with the core points; reorder sections); diff not yet human-reviewed; wording and structure only, factual claims checked against extension.json and src, rendered structure checked after rebase onto the merged #4 and #6.}