bug#46803: User manual does not explain Profiles (nor GUIX_PROFILE)

[Luis Felipe]

The Guix reference manual does not seem to explain what profiles are.


## Steps to reproduce

Assuming you are browsing the Guix reference manual in Info:

1. Type i (to search for an index topic).
2. Type the word profile and hit Enter.
3. Type , repeatedly to explore all the parts of the manual related to profiles.


## Expected result

At some point, I get to a section defining what is a profile exactly, a profile 
of what, what can I get from using one or many of them, what is the 
GUIX_PROFILE variable, does it accept a single path to a profile or more, and 
some use cases and examples.


## Unexpected result

I see profiles are mentioned in places where they can be used, but the text 
assumes I know what a profile is.

Similarly, the GUIX_PROFILE variable doesn't seem to be defined. It seems 
missing from the concept and programming indices.


## Additional information

The "Guix Profiles in Practice" recipe in the cookbook defines profiles to some 
extent. Maybe some of this information can be used in the manual?


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Luis Felipe López Acevedo
https://luis-felipe.gitlab.io/