1.0.0

Rewrite the README as a short, visual introduction

Someone landing on the repository should see what mdmap does in a few
seconds: the banner, one line, two sentences on the two-way sync and
the absence of a separate map file, the demo GIF, and quick links to
install, features, issues, the changelog and sponsoring.

The features are one line each, and a small Trip note shows how
headings, list items and paragraphs become nodes, including two
paragraphs in one node and "Stops:" with its list, and what a new
child writes back. The technical details (settings, the positions
block, embeds loaded from the internet) are kept, shorter. A Support
section links to GitHub Sponsors, and a License section names MIT
and the third-party licenses.