docs: Document macOS font-kit requirement

[huacnlee]

Summary

• Document the macOS font-kit requirement next to the Getting Started dependency setup, in English and Chinese.
• Keep the recommended gpui-kit single-dependency setup: Kit already enables gpui-pre-platform/font-kit.
• Give direct GPUI users copyable dependency entries with the correct feature owner and matching exact snapshot pins, using the site's version variables.
• Explain the NoopTextSystem failure mode and add a feature-inspection command plus Installation troubleshooting links.

Related to #3337. This documents the verified feature wiring; the reporter's manifest and macOS reproduction have not been inspected.

Verification

• bun run test:docs: 8 passed, 0 failed.
• python3 script/check-ai docs: 9 published recipe fragments checked; 6 script tests passed.
• bun script/check-gpui-pin.ts: passed.
• Parsed both locales' TOML examples after version expansion and checked them against the workspace's exact GPUI pins.
• Verified gpui-kit → gpui-pre-platform/font-kit → gpui-pre-macos/font-kit against the workspace and released 0.3.7 manifests; inspected the matching upstream macOS backend and NoopTextSystem implementation.
• Rendered both edited page pairs with the site's Markdown processor; verified expanded versions and both troubleshooting anchors.
• cargo metadata --no-deps accepted the extracted direct-GPUI manifest.
• git diff --check: passed.

Docs-only change. A full native Rust build, macOS visual reproduction, and complete site build were not run in this Linux environment.

AI-assisted documentation change prepared with OpenAI Codex.