Add styling to rels documentation

[bleupen]

The /rels pages are pretty ugly to look at. Add basic styling to the pages. Markdown-based rel documentation can be enhanced just by adding this bower plugin: https://github.com/sindresorhus/github-markdown-css

[felixheck]

@bleupen have you got any hints when this will be enabled?

The current "rels documentation" feature is very rigid, would like to add more customizations. Like e.g. a custom rels overview template.