Improved documentation

dio/dartdoc_options.yaml

@@ -0,0 +1,5 @@
+dartdoc:
+  showUndocumentedCategories: true
+  categories:
+    MigrationGuide:
+      markdown: docs/migration_guide.md

dio/lib/dio.dart

@@ -1,6 +1,7 @@
 /// A powerful HTTP client for Dart and Flutter, which supports global settings,
 /// [Interceptors], [FormData], aborting and canceling a request,
 /// files uploading and downloading, requests timeout, custom adapters, etc.
+/// {@category MigrationGuide}
 library dio;
 
 export 'src/adapter.dart';