linutronix_cmdref: add ln and ctrl+r

The "ln" tool is needed for the basic schulung. Add it to the
command reference handout.

Since adding something to the left column creates empty space in
the right column, use that to add a reference to ctrl+r (searching
the shell history). This is a feature that is always asked about
during the schulung.

Signed-off-by: John Ogness <john.ogness@linutronix.de>

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