Create better documentation of CLI commands

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` ?

Start with the existing command list and the `! nbdev_help` entry point mentioned in the issue. Document links to each associated function, verify command names, add details such as `nbdev_proc_nbs`, explain relationships between commands, and include the markdown representation of the functions.

Written by the indexing model from the issue text.