chore: remove dart_quill_delta directory to move it outside of the repo

[EchoEllet]

Description

Move the package dart_quill_delta from this repo to FlutterQuill/dart-quill-delta

Update the imports in doc/delta_introduction.md. This PR is not a breaking change, the imports were slightly incorrect.

Related Issues

• Related #2259

Type of Change

• [ ] ✨ New feature: Adds new functionality without breaking existing features.
• [ ] 🛠️ Bug fix: Resolves an issue without altering current behavior.
• [ ] 🧹 Code refactor: Code restructuring that does not affect behavior.
• [ ] ❌ Breaking change: Alters existing functionality and requires updates.
• [ ] 🧪 Tests: Adds new tests or modifies existing tests.
• [x] 📝 Documentation: Updates or additions to documentation.
• [x] 🗑️ Chore: Routine tasks, or maintenance.
• [ ] ✅ Build configuration change: Changes to build or deploy processes.

[EchoEllet]

Note that we should include a reference to or indicating that Quill Delta is not a format specific to flutter_quill and can be used outside of Flutter and Dart somewhere in the README.md since the main reason that users do want to convert and store data in HTML is that they want something more future proof in case they want to migrate to another package or framework. There are many packages in Dart and other language to support Quill Delta.