chore: drop the npm publishing pipeline and rewrite the README [skip ci]

Everything removed here served the old model: one branch per architecture,
built by scripts/{setup,build,patch,archive}.sh and published to npm as
v8-android* packages. The matrix in scripts/matrix/ replaces the build
side, and the artifacts now ship as GitHub release assets -- npm buys
nothing when the only consumers are two build scripts we control, and a
published package implies a support surface that does not exist.

Removed:
  scripts/{setup,build,patch,archive,env,start}.sh, bump.py, publish.py,
  export_npm_env.sh   superseded by scripts/matrix/
  packages/, lib/, package.json                   npm distribution
  .github/workflows/{android,ios}.yml, Dockerfile superseded
  mkcodecache/                                    unreferenced; the runtimes
                                                  build code caches at runtime
                                                  via ScriptCompiler
  ten stale patches                               only the finalizer and
                                                  android build patches are
                                                  still applied

The README was still Kudo's react-native-v8 text. Rewritten for what this
repo now does, with the upstream credit kept. scripts/matrix/README.md is
folded into it so the two cannot drift.

Author: edusperoni