Restructure o1js docs for clearer entry points

[garwalsh]

As detailed here: https://www.figma.com/file/DpcLzFDA0dfZuTQSN48eff/o1js-Docs-Structure?type=whiteboard&node-id=0%3A1&t=tn2wTMXL654GfCMu-1

Restructure o1js docs so that circuit writing and zkApps material is separated such that the developer journey is clearer