Add chapter about CSS

[ThijsTyZ]

This chapter is not finished yet. Please complete before merging.

Added by @ThaNarie; topics to cover (feel free to edit)

• [ ] use of seng-scss
• [ ] use of CSS in different frameworks (SCSS, CSS modules, styled components)
• [ ] naming conventions
• [ ] style-lint reference (and add in separate PR) (which include most of the formatting/nesting/ordering/etc rules)
• [ ] use of media queries
• [ ] use of z-index
• [ ] use of feature detection
• [ ] how to organise your styles/files
• [ ] when to (not) use inline styles
• [ ] how/when to use css animations
• [ ] css performance (positioning, will-change)
• [ ] use of !important
• [ ] use of em/rem/etc sizing/scaling
• [ ] how to deal with fonts
• [ ] mediaqueries

[pigeonfresh]

I have updated this PR with the latest updates and added an introduction instead so we can add topics and examples.