Create better documentation of CLI commands

[hamelsmu]

A single list of all the commands, but with the following improvements

1. A link to the associated function (should also make sure the names match the command)
2. A bit more detail in some cases about what each thing does ex: nbdev_proc_nbs
3. How each command relates to eachother

Can we display the markdown representation of all these functions in addition to running ! nbdev_help ?

[castorfou]

is it possible to add a command nbdev_version or specify the nbdev version in nbdev_help
that may be useful