Deprecate nuxeoctl register-trial command

Nuxeoctl's register-trial command is:

• barely used by our customers
• difficult to take care of
  • it's a marketing-driven feature but it can't be updated through a hotfix
  • marketing can want to adapt it anytime, keeping compatibility for 3 years is possible but has high impacts on marketing team's work
• half useful since you can't validate your trial using the command line anyway
• command line is really useful to automate deployment, but trial registration is not something you want to automate
• not used in internal tools, and that could be better replaced by a REST API endpoint if needed by cloud team or others

=> deprecate that command

TODO

• when using nuxeoctl register-trial (with or without arguments), nuxeoctl should display the following text:

  This command is deprecated. To register for a free 30 day trial on Nuxeo Online Services, please visit https://connect.nuxeo.com/register

• nuxeoctl help should display a deprecated mention on this command.
• Update documentation page nuxeoctl and Control Panel Usage