docs: refresh the README for GitHub, npm and NuGet

[Danielku15]

Note

AI-authored disclosure (alphatab-ai-authored-v1)

Portions of this content were authored by an AI agent. The agent has read
AGENTS.md and the human submitter accepts responsibility for
compliance with the rules in that document.

Issues

Fixes #

Proposed changes

The README is the landing page on GitHub, and the same file ships verbatim in the npm package and both NuGet packages. The current one is outdated: it lists Guitar Pro 3–7, leaves out Capella, has no code sample and still shows netcoreapp3.1 platform rows.

This PR restructures it as a modern landing page:

• First screen: tagline, a two-sentence pitch, version and download badges for npm, NuGet and Maven, and a link row (website, docs, Playground, features, release notes, discussions).
• Hero image: bars 77–80 of the Iron Waltz demo song, rendered by alphaTab in a player window with a loop range, playback cursor and the backing track's waveform. It links to the Playground. The image is a single self-contained SVG (66 KiB, 19 KiB gzipped) that follows the viewer's light or dark theme through prefers-color-scheme. All text and music symbols are outlined, so it needs no fonts wherever it is embedded.
• Hero generator: npm run readme-hero -- --song <file> regenerates img/hero.svg. It renders the passage with alphaTab in Node through a custom render engine. That engine outlines text and Bravura symbols with the fonts bundled in packages/alphatab/font and measures text with the same fonts, so layout and outlines match. Timing, section labels and the waveform come from the song and its backing track (decoded with ffmpeg). opentype.js (MIT) is added as a root devDependency for the outlines. The song itself is not part of the repository.
• Content: highlights, supported read and write formats, a quick start with npm install and a runnable CDN snippet, a platforms table, the packages in this repository, documentation links (including llms.txt), community links including Show and tell, licensing of the bundled fonts and SoundFont, and thanks.
• Rendering everywhere: plain Markdown with absolute URLs only. nuget.org strips HTML and only loads images from allowlisted hosts such as raw.githubusercontent.com, and npm ignores <picture>, so the file renders the same on all three.
• npm listing: new description and keywords for @coderline/alphatab.

The hero URL points at develop, so the image only appears in the rendered README once this is merged.

Checklist

• I consent that this change becomes part of alphaTab under its current or any future open source license
• This PR is linked to an accepted issue (see above)
• Changes are implemented
• New tests were added
• I have read AGENTS.md if an AI helped draft any part of this PR

AI authorship disclosure

• No AI agent authored any part of this PR (description, code, tests, or commit messages)
• An AI agent contributed to this PR. The AI-authored disclosure block
  (alphatab-ai-authored-v1) is present at the top of this body, and I have
  personally reviewed every change and can explain each one

Further details

• This is a breaking change
• This change will require update of the documentation/website