Update Creating custom traits tutorial - https://documentation.magnolia-cms.com/display/DOCS/Creating+custom+traits

Explain in more detail how to implement the converterClass - according to [~jschulte] this point is missing on the current tutorial!

The page must show / describe what we provide with the module documentation-trait-tutorial.
Screenshots are taken from content which is bootstrapped by the documentation-trait-tutorial module.

Think about whether we should this time create variations of components instead of pages. (cmeier votes for components, but we may also ask P.D. about this.)