Docs: Improved Regtest Setup Instructions

[kilrau]

Divide into legends for integrations, https://docs.boltz.exchange/v/api/regtest for back end development.

Bull Bitcoin feedback re legends environment:

• install docker-compose-plugin. Pretty easy to find instruction in net.
• add dockerd service is systemd.

sudo systemctl enable docker.service
sudo systemctl start docker.service

• Don't miss the source docker-script.sh step.

• to use lightning, do the lightning-cli-sim 1 <command>. Putting a 1 there was really un-intuitive until i looked inside the bash.