Convert to jekyll

[Vanuan]

Gov.uk is currently using RoR, even for static content.
It might be possible to convert static parts to jekyll/markdown, to ease editing and translation.

[Vanuan]

Conversion of index page is in progress:
https://github.com/Maidan-hackaton/design-principles/blob/gh-pages/index.md

Probably, it is reasonable to sacrifice the ease of web-design to the ease of content edition.

[Vanuan]

index design-principles page is finished.