Add Telegraf commands and flags to docs

[sjwang90]

Telegraf commands and flags aren't documented anywhere. I have a PR to add them to the GitHub docs but they should be in the customer facing docs as well.

GitHub docs:

• https://github.com/influxdata/telegraf/blob/ab15e5797bcb80e4f44df3f8d09bf8e5267a8a1e/docs/COMMANDS_AND_FLAGS.md

Something similar to what we have for influxd:

• https://docs.influxdata.com/influxdb/v2.0/reference/cli/influxd/

[sjwang90]

This just got merged into GitHub docs: https://github.com/influxdata/telegraf/blob/master/docs/COMMANDS_AND_FLAGS.md

Can pretty much copy what's in there