Update docs to 0.8

[ethanfrey]

• [x] Intro/theory docs
• [x] Update "getting started"/first contract
• [x] Update deployment instructions to use demo net and faucet, not localhost.
• [x] Add deploy instructions using @cosmwasm/cli not just wasmcli
• [ ] Independent review of new "getting started" flow

[ethanfrey]

Merging this now. Happy for someone to try it out and give me feedback - make comments here and I will revisit it.