docs: surface support matrix + system requirements in README

[ubugeeei]

Problem

README mentions Node + npm versions only. Users don't know whether macOS Intel, Linux ARM, or Windows ARM are supported — and the build infers only macOS arm64 / linux x64 / win32 x64 via `getPlatformKey`.

Expected

A short "System requirements" block in README listing:

• Supported OS/CPU tuples (Linux x86_64, macOS arm64, Windows x86_64).
• Required external binaries (ffmpeg, Chromium come bundled).
• Hardware notes (GPU, RAM, disk).

Acceptance

• README has a clearly-labeled section.
• `docs/docs/production.md` cross-links it.