Adds Nix CLI Guideline to docs

As we are working towards Nix 3.0 we want to make sure that we make a
huge step forward in Nix's user experience. And once 3.0 is out of the
door we need to make sure that all future commands and features keep up
the standard of user experience.

This PR adds a CLI guideline document to the Nix documentation. Consider
this document a good starting point and a checklist when somebody will
be (re)implementing commands.

Clearly this guideline does nothing to improve user experience on its
own and can only be useful as long as it is going to be read and
cared for. But it is a first step into that direction.

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