Document `maestro validate` commands

[JenySadadia]

Add documentation for maestro validate command. The command is used to validate maestro results with the dashboard results.

[aliceinwire]

thanks for opening this issue, as we already discussed on the weekly KernelCI meeting I will add the documentation in the next few days before releasing 0.1.9

[aliceinwire]

currently writing, sending pull request next week

[aliceinwire]

#242