message: added selector links to cheatsheet
diff: * **[-sb-mixin](./mixin-syntax.md)**: apply mixins to CSS ruleset
  * **[-sb-variant](./variants.md)**: apply pre-made look and feel
  * **selectors**
  - * **[]()**:
  - * **[]()**:
  - * **[]()**:
  - * **[]()**:
  + * **[Pseudo-elements](./pseudo-elements.md)**: `::X` target internal part
  + * **[Pseudo-classes](./pseudo-classes.md)**: `:X` target state
  + * **[Custom selectors](./custom-selectors)**: `:--X` selector shortcut
  + * **[Global selectors](./global-selectors)**: `:global(X)` keep selector global - not scoped
  