docs: add documentation for recommended third-party packages

doc/docusaurus/docs/3_other_notable_packages.md

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+---
+sidebar_label: Other notable packages
+sidebar_position: 3
+---
+
+# Other Notable Packages
+
+Notable third-party tools that complement `solid_lints` in keeping Dart and
+Flutter codebases clean, robust, and maintainable.
+
+## [undead](https://pub.dev/packages/undead)
+
+[![pub package](https://img.shields.io/pub/v/undead.svg)](https://pub.dev/packages/undead)
+
+Deterministic reachability and dead/unused declaration analysis for Dart and
+Flutter packages.
+
+It performs whole-package AST analysis using `package:analyzer` to build a
+reachability graph from known entrypoints to all internal declarations,
+identifying unused top-level declarations, classes, functions, and variables.
+
+- **Links**: [pub.dev](https://pub.dev/packages/undead) ·
+  [GitHub](https://github.com/kevmoo/analytica.dart/tree/main/packages/undead)
+
+---
+
+## [cognitive_complexity](https://pub.dev/packages/cognitive_complexity)
+
+[![pub package](https://img.shields.io/pub/v/cognitive_complexity.svg)](https://pub.dev/packages/cognitive_complexity)
+
+Algorithmic Cognitive Complexity calculation and Data-Flow analysis library and
+CLI tools for Dart and Flutter.
+
+It implements the Cognitive Complexity principles articulated by SonarSource,
+providing an objective way to find and fix overly complex logic and routines.
+
+- **Links**: [pub.dev](https://pub.dev/packages/cognitive_complexity) ·
+  [GitHub](https://github.com/kevmoo/analytica.dart/tree/main/packages/cognitive_complexity)