This fixes the organization of the monitoring docs
and the current build errors.
* Delete `overview.md` and move its content elsewhere
* Moved InfluxQL commmands to new page, "Diagnostics"
* Move note on Influx Aware to index page.
* Remove content monitoring with `_internal`, since it's not recommended.
* Add intro paragraph to index and fix weights
* Delete doc with duplicate content
Closes#3193.
pierwill