Add a Styleguide

We should add a Styleguide for t3olayout to have a clear definition of available variables, components, layouts etc.

To Do:

• Set up Styleguide using kss-node
• KSS comments to already existing components Set up navigation with section file headings
• Adjust current styles to match with the TYPO3 Web Styleguide Probably too ambitious for a single Merge Request, also the SCSS requires a lot of rework and restructuring