Restructure the ts dev proxy guide opening into an explicit onboarding flow

[aram356]

Context

docs/guide/ts-dev-proxy.md was streamlined to use the installed ts binary and
now opens with a macOS-only note, install/update, CA trust, and a complete Chrome
quick start. A deep review suggested going one step further: give the opening an
explicit, copy-paste-friendly structure so a first-time reader can follow it
top-to-bottom without hitting advanced material mid-flow.

Proposal

Restructure the opening of the guide into four clearly labeled steps:

1. Prerequisites — call out macOS as a hard requirement (ts dev proxy only
   exists on macOS builds), plus the Rust toolchain and a supported browser.
2. Install or update — cargo install-cli, PATH note, re-run after pulling
   CLI changes.
3. Trust the development CA — ts dev proxy ca install.
4. First run — one complete Chrome example.

Then move the advanced forms (no-launch / manually pointing a browser, multi-browser
--launch, --map vs -f/-t, --resolve, non-loopback listen) into a later
"Advanced usage" section so they don't interrupt the onboarding path.

Acceptance

• The opening reads as a single copy-paste onboarding sequence.
• Advanced options are reachable but no longer sit between install and first run.
• No behavioral/CLI claims change — this is a structure-only pass.

Notes

Originated from deep-review feedback on the dev proxy guide. Docs polish, low
priority.