Update demo docs

[CodeHeister]

Update the Demo documentation for the TypeScript / SolidJS rework

The pages were written with AI assistance (Claude) and reviewed and directed by the maintainer. The text was checked against the source code of the reworked Demo.

Summary

The Demo was rewritten from vanilla JS/DOM into a TypeScript and SolidJS application (Avdan-OS/Demo). The old documentation in docs/demo described the previous implementation (libdemo, libwin, libdrag, Window.make, Keylogger.press, ...), which no longer exists.

This PR replaces docs/demo with a new documentation for the reworked Demo. The URL of the intro page (/demo/demo-intro) is kept, because the Demo itself and its README link to it.

What changed

Removed: the whole previous documentation of the Demo (72 files) and its image static/img/demo-window.png, which nothing else uses.

Added: 26 files (22 pages and 4 category files), in this order in the sidebar:

Section	Pages
Intro	demo-intro — what the Demo is, what is inside, where to go next
Getting started	Installation and scripts, project structure and the dependency rule, deployment to GitHub Pages
Usage	Windows, tabs, dock, start menu, workspaces, language, context menu, keyboard shortcuts
How it works?	Logic behind the Demo, windows, apps, translations
Guides	Add an app, add a language, add a track, add a lib module
Library reference	i18n, keylogger, workspace, context menu, drag and drop, player, theme / assets / start menu, window store

How it was checked

• The behaviour and the API were taken from the TSDoc comments and the code of the reworked Demo, not from the old docs. Two mistakes found this way were fixed before opening the PR (the snapping behaviour at the top screen edge and the list of drag kinds).
• The site builds with docusaurus build, with onBrokenLinks: 'throw' enabled: no broken links, all 26 pages are generated.
• The pages were reviewed visually in the site theme.