Adjust documentation to deposit removal

[fhildeb]

This PR updates the documentation and a terminal output for the deposit removal:

• Updates deposit-cli exclusivity in lukso import notice
• Removes deposit command and examples from README.md
• Updates validator-keys to --keys-dir
• Improves documentation for validator explanation
• Adds flag notices to lukso start and lukso validator commands
• Adds Prysm validator specification to external link list

[github-actions[bot]]

:rocket: Deployed preview to curl https://install.lukso.network/73 | sh