Document `DISABLE_CODE_INSIGHTS`

Created by: daxmc99

We have several customers running with DISABLE_CODE_INSIGHTS=true

Currently, this is an undocumented feature but since it is in our codebase, we should either remove it or fully support it.

Documentation of this feature will include where to place this env var as our current tribal knowledge says to place it on frontend and gitserver.

What are the effects of this env var being set?