[RELAY][DOCS] Relay Documentation Sprint #2212

Now that we are working towards a beta version of Relay and have stabilized many of the APIs it would be good to sprint on completing a first version of the documentation for the APIs and IR.

We have a couple pieces of docs:

• Tianqi's tutorial (see [DOCS] Introduction to Relay IR. #2185)
• API Docs (see [DOC][Relay]: Add API docs for Relay. #1750, @jroesch is in process of updating it)
• Language Reference (will be derived from [DOC][Relay]: Add API docs for Relay. #1750).
  • Add introduction to Relay to language reference.
  • Add language reference for the expression language.
  • Add language reference on type system.
  • Incorporate the operator index.
  • Add list of optimization and brief descriptions.
• Developer's guide to Relay
  • Add introduction
  • Explain System Arch.
  • Explain type inference
  • Pass architecture
  • Technical cover of optimizations
  • Platforms
  • Port ([DOC] Codebase walkthrough with Vector add example #2206)

Will continue to update with more fine grained goals as we open PRs.