Docs/add documentation

[Nenuphar12]

Main changes :

• adds documentation for StravaClient and most repositories

Minor changes :

• upgrades .gitignore to Flutter's default one and removes unnecessary files (mostly generated files)
• adds intent-filter for flutter_web_auth in example/android/app/src/main/AndroidManifest.xml
• some minor formatting in the modified files

[dreampowder]

Wow, that was something that i always wanted to do but never had time to do so. Thanks a lot👌

[Nenuphar12]

My pleasure ! 🍻