Add DEPLOY.md: ports, firewall, RPC credentials, SSL, operation

Also fix the COIN default in docs/environment.rst, and refer to the Palladium
node setup by concept instead of by paths of its repository.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

docker-compose.yml

@@ -1,4 +1,4 @@
-# ElectrumX for Palladium, next to the node of palladiumcore/docker.
+# ElectrumX for Palladium, next to the Palladium node container.
 # Start the node first: it creates the "palladium" network. Setup and usage: README.md
 services:
   electrumx: