Simplify button docs and reduce/improve modifiers

[davidknezic]

Issue first raised in pr #111.

I'd suggest simplifying the button docs, there is too much technical talk there about "Margins and paddings have been changed" and "Border-Thickness has been increased to 2px on default and large size".

Also I'd like to see a system for the modifiers, which is more easily described, for example:

Primary button

This one can only be blue. It can contain icons. It's meant to be placed on plain, light backgrounds.

Secondary button

This one always has a transparent background! It comes with blue or white strokes, depending on where it's placed. It's meant to be placed over pictures and dark backgrounds.

Tertiary button

This one comes in blue and orange. Blue is standard, orange is for ... (?).

[FKSI]

@davidknezic Many thanks for the PR!

For the Secondary Buttons we still need two modifiers don't you think?