docs: troubleshoot the Xcode FLUTTER_ build setting override warning

[AbhishekDoshi26]

Since CLI 1.6.94, Shorebird warns when an Xcode project hard-codes FLUTTER_* build settings. The docs never explained that warning, so this adds a troubleshooting entry.

What it covers

• When it shows up: shorebird doctor, shorebird init, and iOS / iOS-framework release and patch. It's a warning, not an error, so builds aren't blocked. Taken from iosCommandValidators and initAndDoctorValidators in doctor.dart, and from validatePreconditions, which only fails on errors.
• The message: matches the ValidationIssue text in xcodeproj_flutter_override_validator.dart.
• Why it matters: Flutter sets these in Generated.xcconfig, and a value in the Xcode project overrides that file. A hard-coded FLUTTER_ROOT, for example, can make Xcode build part of the app with a different Flutter than Shorebird's. That's the failure the validator's doc comment describes.
• How to fix: remove the settings under Build Settings in Xcode.
• Exceptions: FLUTTER_TARGET is allowed (flavor entry points), and Flutter modules are checked under .ios/ instead of ios/.

Also adds pbxproj to the shared cspell word list.

[AbhishekDoshi26]

Reviewed against xcodeproj_flutter_override_validator.dart and doctor.dart on shorebird main. One finding:

The intro leaves out shorebird init. The validator is in initAndDoctorValidators, and init_command.dart runs that list at the end of shorebird init. So a new user sees this warning first from init, not doctor. The PR description already lists init. Only the page is missing it.

The rest checks out: the message text matches the ValidationIssue, it's a warning (ValidationIssueSeverity.warning), FLUTTER_TARGET is the only exemption, and modules are scanned under .ios/. The validator also runs for iOS-framework release and patch (ios_framework_releaser.dart, ios_framework_patcher.dart), which "for iOS" covers well enough.

[AbhishekDoshi26]

Pushed cacbb13. The intro now names shorebird init alongside shorebird doctor.