Add a proper documentation

[diegoferigo]

Edit: I converted this issue to be more generic. Every time we find something that still needs to be documented, we can reference this issue.

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As #36 (comment) and #36 (comment), we should provide reference documentation page we can link every time we need to refer to the notation used in our code.

• Link velocity (both linear and angular parts)
• Link orientation
• External forces
• Contact forces

[diegoferigo]

A first draft of a sphinx-base website has been implemented in #195. They website is still empty and I leave this issue open to track the progresses.