Splitting intro into two tutorials

[connorcartwright]

Fixes #658 Closes #661

Hey guys @AurelioDeRosa @agcolom @gnarf @arthurvr

I'd like to continue on from #661 and hopefully get these new pages setup soon - I'll be giving them a good read through tomorrow (and double checking that all issues raised have already been amended) but any edits we want made I can do ASAP 👍

I've made a minor amend (fixing references to main.js that should have been demo.js) and hope to improve the files as much as possible.