Documentation: Basics of command-line usage

[Lestropie]

To be ported to the MRtrix3 documentation:

https://jdtournier.github.io/cmdline-tutorial/index.html

Probably as a stand-alone page; the current "Command-line usage" page would then be more "advanced" usage / usage specific to MRtrix3.

[jdtournier]

Note I've ported this over to readthedocs - reST sources for this live in this repo, and all MRtrix3 team members have access.

[Lestropie]

@jdtournier: Given your effort to fill out the command-line basics tutorial, I think it's worth adding links to it in the main repo documentation wherever appropriate.

[jdtournier]

closed - now available at https://command-line-tutorial.readthedocs.io/

[jdtournier]

Reopening - still need to refer to the tutorial from the main documentation.