chore: drop the open licence and stop taking pull requests

The site exists to be read, not reused. It is no longer licensed for
anything, and external contributions have moved to issues only.

- Replace the MIT + CC-BY-4.0 dual licence with an all-rights-reserved
  notice. No permission is granted to copy, modify or distribute any part
  of the repository; reading and linking stay free.
- Carve out third-party components explicitly. Rspress, React, Rsbuild and
  Tailwind CSS keep their own licences, and an all-rights-reserved notice
  that did not say so would be wrong.
- Rewrite CONTRIBUTING around issues rather than pull requests, and say
  plainly that PRs will be closed without review.
- Mark the package UNLICENSED, the npm convention for proprietary code.

Note: this makes the repository source-available, not open source — a
licence that forbids all reuse cannot meet the OSI definition. The README
no longer calls it open source.

Closing PRs for real also needs the "Pull requests" feature switched off
in the repository's GitHub settings; that is not something this commit can
do.

package.json

@@ -2,6 +2,7 @@
   "name": "onixbyte-homepage",
   "version": "1.0.0",
   "private": true,
+  "license": "UNLICENSED",
   "type": "module",
   "packageManager": "pnpm@12.5.1",
   "engines": {