wp-docs

Level 0 : Learn about WordPress
Level 1 : Install / update WordPress
Level 2 : Configure WordPress
Level 3 : Enhance the appearance (themes)
Level 4 : Enhance the behavior (plugins)
Level 5 : Modify the code
Level 6 : Modify the database
Level 7 : Create new themes
Level 8 : Create new plugins
Level 9 : Improve the performance