Move style guide to its own folder

doc/development/README.md

doc/development/contributing/index.md

doc/development/contributing/style_guides.md

doc/development/documentation/feature_flags.md

doc/development/documentation/graphql_styleguide.md

doc/development/documentation/index.md

doc/development/documentation/restful_api_styleguide.md

doc/development/documentation/structure.md

doc/development/documentation/styleguide.md

doc/development/documentation/styleguide/index.md

doc/development/documentation/testing.md

doc/development/documentation/workflow.md

doc/development/ee_features.md