a manpage for envml command

[cmarqu]

The envml tool that ends up in Modules/bin/ seems quite useful but does not show up in the documentation (apart from the Release Notes: https://modules.readthedocs.io/en/latest/NEWS.html?highlight=envml).

[xdelaruelle]

It should get its own man page: envml.1

[xdelaruelle]

Some guidelines for people willing to contribute on this issue:

• a envml.rst documentation file should be added to the doc/source repository
• this file should have the same structure as doc/source/ml.rst document (with a SYNOPSIS, DESCRIPTION, EXAMPLE, EXIT STATUS, ENVIRONMENT and SEE ALSO sections)
• look at script/envml script to learn how this shell command (similar to env command) operates
• script/envml script also provides some examples in its help message