Start the contact generation, bounding volume and ray cast chapters.
Fill other parts + improve the download buttons css.
`How to contribute` and `FAQ` are mostly done.
Add several code samples and explain the differenciation DSL.
The concept of margin has been removed from ncollide: adapt the examples.
Complete all parts, including examples and illustrations.
Move the gitbook to the root directory.

This completely removes the old website.
Replace all <div> by <span>.
Fix some typos.
Minor adjustments of some figure styles.