docs: revamp root README into a landing page

[Lyokone]

Summary

Rewrites the root README.md from a bare package list into an attractive GitHub landing page, aimed at making the repo more welcoming and driving adoption.

The previous root README was ~30 lines and — via a leftover copy-paste — actually mis-described the plugin as one that "display[s] local notifications within Flutter applications."

What's new

• Hero section — emoji title, one-line pitch, and the six supported platforms up front.
• Badge row — pub version, pub likes, pub points, codecov, license.
• Quick-nav links — Docs · Live web demo · pub.dev · Feedback.
• "Why Location?" feature highlights.
• Platform support matrix broken down by feature (one-time / stream / background / permissions), accurately reflecting that background updates are Android + iOS only.
• YouTube demo thumbnail (carried from the package README).
• Copy-pasteable quick start — a minimal, correct snippet (service check → permission → one-time fix + stream).
• Packages table with per-package pub badges.
• Contributing section, maintainers, and MIT license.

Docs-only change — no code touched.

[cedvdb]

@Lyokone FYI the web demo has a 404