Dev mode docs and defaults

[riongull]

Dev mode could use a little more documentation.

There's an option in the UI (Networks > Advanced Settings > Configurations) to toggle dev mode, stating that it "enables advanced features and less strict validation", but I don't find any reference to dev mode in either the dash.org docs, or the README.md in the repo to learn more.

Also, the default configs for dev mode in .env are:

MAINNET_developer_mode=true
TESTNET_developer_mode=false

Why these defaults? I would expect them to be opposite.