docs: document component-agnostic styles

Closes #616 (closed)

This adds some documentation to explicitly state that we expect utility mixins to be component-agnostic and that, in cases where adding component-specific mixins can't be avoided, they should be named in a way that makes them clearly identifiable as ~"technical debt" so that we can easily convert them to generic mixins at later development stages.

Additionally, all current component-specific mixins have been renamed to include the gl-tmp- prefix.