docs: beginner-friendly Quick Start guide for Linux/Windows/macOS (#414)

Adds docs/quickstart.md — a step-by-step, no-experience-assumed walkthrough
from installing the build tools to the first coli chat, with per-OS
copy-paste commands (Ubuntu apt, Windows MSYS2 or prebuilt binary, macOS
brew), the ready-made HF int4 container plus the self-convert path, and an
honest 'what to expect' on disk-bound speed. Commands verified against
setup.sh and the coli subcommands; every cross-linked doc exists. Linked
from the README's Get started section.

Closes #414

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

README.md

@@ -151,6 +151,10 @@ scale-granularity/rotation ablations live in
 
 ## Get started
 
+> **New here?** The [Quick Start guide](docs/quickstart.md) walks through
+> install → build → model → first chat step by step for Linux, Windows, and
+> macOS, with copy-paste commands and no assumed background.
+
 ### 1. Get the model
 
 A pre-converted **GLM-5.2 int4** container is on Hugging Face — **use the