documentation: add osd erasure-code-profile {set,rm,get,ls}

And a link from rados/operations.

http://tracker.ceph.com/issues/8514 Fixes: #8514

Reported-by: Kenneth Waegeman <Kenneth.Waegeman@UGent.be>
Signed-off-by: Loic Dachary <loic@dachary.org>