Document usage of enable-elytron.cli script

With JBEAP-8472 was introduced a new example configuration script which enables Elytron as security provider across the server. The documentation should mention it and provide sample usage.

The script lives in directory docs/examples in the server installation.

One can use it by (Linux way):

$JBOSS_HOME/bin/jboss-cli.sh --file=$JBOSS_HOME/docs/examples/enable-elytron.cli