docs/???: Injecting system values into an evaluation

[jpluscplusm]

A guide that shows examples of -T being used with the different cue sub-commands that accept it.

This could take the form of a concept guide, a howto guide, or a reference page complementing cue help injection. TODO: decide which form makes sense.

Demonstrate:

• the different cue sub-commands that accept -T
• the result of -T for each variable that's available; potentially combined with defaults to show fallback logic in action
• overriding system variables with an explicit -t

Related: #145

[jpluscplusm]

Closed via https://cuelang.org/cl/1196597.