docs(spm): mark preview, fix fresh-clone/CI + stale VFS, SwiftPM naming

Overhaul the SwiftPM tooling docs to match current behavior:

- Mark the feature Preview (header + intro) — commands/layout/distribution
  may change; CocoaPods stays the default.
- Fresh clones & CI: document that Xcode resolves the package graph before
  any build phase, so a clean checkout needs one `npx react-native spm` run
  (or a `postinstall` hook) before xcodebuild — the auto-sync phase can't
  bootstrap it. Correct the Quick Start lines that implied otherwise, and add
  a "Resolve Package Graph" row + the matching Troubleshooting entry.
- Drop stale VFS overlay references (VFS was removed; headers now come from
  the React / ReactNativeHeaders / ReactNativeDependenciesHeaders products) —
  rewrite Header Resolution, and fix the sync-spm-autolinking.js comment.
- Directory Layout: list all five artifacts.
- Migration: spell out exactly what `--deintegrate` removes vs preserves and
  how to keep non-RN pods (re-`pod install`, open the .xcworkspace, never
  re-add `use_react_native!`).
- New Brownfield section (project must live inside the JS tree; use
  `--xcodeproj`/`--productName`; native-root-with-RN-subfolder unsupported).
- New "Community packages without a Package.swift" section — scaffold +
  patch-package, and recommend filing an issue/PR to ship the manifest
  upstream.
- Use "SwiftPM" in prose throughout (identifiers, the `spm` command, and the
  literal "Sync SPM Autolinking" phase name kept verbatim); move the
  Pipeline/Directory/Header/Auto-Sync deep-dives into a Reference section.

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

Author: chrfalch