LU-930 doc: add lnet man page

LU-930 doc: add lnet man page

Add a simple lnet man page which provides a short description of
lnet and points to all lnet related commands, like lnetctl and lst.

This page will also help users find more lnet related pages using
apropos.

Test-Parameters: trivial
Signed-off-by: Timothy Day <timday@amazon.com>
Change-Id: Ia3c1be4e60118e00ab9378ef7cd2d53b25196cf2
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/51085
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Frank Sehr <fsehr@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>