[doc] add a link h2olog in "Key Features"
[doc] add a link h2olog in "Key Features"

h2o/h2o,cwyang/h2o,cwyang/h2o,yannick/h2o,yannick/h2o,yannick/h2o,h2o/h2o,cwyang/h2o,devnexen/h2o,h2o/h2o,yannick/h2o,h2o/h2o,yannick/h2o,h2o/h2o,devnexen/h2o,yannick/h2o,devnexen/h2o,h2o/h2o,h2o/h2o,devnexen/h2o,cwyang/h2o,devnexen/h2o,devnexen/h2o,devnexen/h2o,h2o/h2o,yannick/h2o,cwyang/h2o,yannick/h2o,yannick/h2o,devnexen/h2o,cwyang/h2o,h2o/h2o,cwyang/h2o,devnexen/h2o,cwyang/h2o