Add setup instructions for bitstyles

[andreasknoepfle]

We should add some nice instructions on how to use bitstyles in phoenix projects with webpack.

For this, we decided that we want to outline the setup for the full version of bistyles with all things imported, but still with the option to set variables.
Also add some setup instructions for svg icons & fonts and a way how to purge unused css.

• add bitstyles sass setup instructions with webpack (in phoenix) --> check out bitstyles, there is some ideas
• add bitstyles sass setup instructions with esbuild (in phoenix)
• setup instructions for svg icons (see Allow configuring icon file #45)
• setup instructions for fonts