v0.2: mdBook initial version

Pan Docs rendering pipeline has been successfully moved to mdBook, along with a number of infrastructural changes.

Thanks and congrats to @ISSOtm for preparing this migration and working on making the mdBook version feature-equivalent with the old one.

A number of the features are also provided by this new frontend:

• single and multi page layout
• themes, palettes, better theming
• support for custom backends
• custom language highlight.js
• fixes for #289, #290, #7, #19, #58, #60

More info in PR #292