Bisq developer docs

CONTRIBUTING.md: Understand Bisq's contribution and compensation guidelines
build.md: Build and run Bisq at the command line
idea-import.md: Import Bisq sources into IntelliJ IDEA
Makefile: (new) Set up a self-contained local Bisq network on Bitcoin regtest
dev-setup.md: (deprecated) Set up a self-contained local Bisq network on Bitcoin regtest
dao-setup.md: (deprecated) Set up a complete Bisq DAO development environment
testing.md: Learn about the Bisq testing process and how you can contribute.

Looking for user-facing documentation? See https://bisq.wiki.