Update detekt extensions doc (#1893)
Update detekt extensions doc (#1893)

* Update detekt extensions doc 

Closes #1864

The old extensions documentation included deprecated code samples.
This deprecated code samples were replaced with the code in the 
detekt-extension project.

* Apply suggestions from code review

Co-Authored-By: Nicola Corti <853e45f35563f7cce7710b9a49c571722345c5bb@gmail.com>

* Add THRESHOLD constant to code sample

rock3r/detekt,arturbosch/detekt,rock3r/detekt,rock3r/detekt,arturbosch/detekt,arturbosch/detekt