Implement new documentation structure.

[cwaring]

Once an agreed documentation structure has been finalised we will need to schedule a temporary code-freeze for contributions in order to implement the new structure. For this to be possible all existing PRs must be finalised and merged/closed to minimise the risk of conflicts.

Related to #159

[pooja]

I believe this issue is now closed, since we have implemented the new documentation structure at a high-level. Please reopen if you believe there is more left in this issue.