Add documentation on --config-file and CABAL_CONFIG env variable.

[m-renaud]

Add information about the --config-file command line flag and the CABAL_CONFIG environment variable for specifying the cabal config file to section 2.1 (Configuration) of the Cabal User Guide.

Fixes #5322.

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[hvr]

I wonder if it would make sense to create a special section in https://cabal.readthedocs.io/en/latest/cabal-projectindex.html for special CLI flags such as --config-file which don't fit into the existing sections.

[23Skidoo]

Merged, thanks!

[23Skidoo]

@hvr

I wonder if it would make sense to create a special section [...]

SGTM