ABI: Prep user guide for initial ABI release

[michaeldiamant]

Problem

We're aiming for our 1st ABI release, which brings numerous ABI capabilities to production. See associated epic: #185.

Since we haven't developed a user guide in parallel with each feature, the story asks for:

• A user guide priming PyTeal developers to leverage ABI functionality:
  • Router method definition + registration
  • Contract description (JSON) generation
  • ABI type read + write usage
• Explicit mention for limited level of support mirroring disclaimers around Router and ABIReturnSubroutine.

Solution

Dependencies

Urgency