docs: update operator guide

[Mirko-von-Leipzig]

This PR refreshes the network operator guide of our public docs.

Similarly to #2185, I recommend reviewing the docs in-browser:

make install-tools
cd docs/external                                                                                                                                      
npm ci
npm run start:dev

In addition, this PR also:

• Publishes remote prover and network monitor docker images
• Renames validator env vars from MIDEN_NODE_VALIDATOR to MIDEN_VALIDATOR
• Standardizes the validator key CLI

[Mirko-von-Leipzig]

I'm going to merge these so the live docs are shareable; I'll address review comments as follow-ups.