Implement a setting directive in the documentation

[Gallaecio]

Scrapy has a setting directive implemented for Sphinx documentation that allows linking to settings while formatting them as code in an easy manner.

Looking at #212, I think Spidermon could benefit from implementing such a directive as well.

[vipulgupta2048]

Hey @Gallaecio, sounds interesting. Can you point to where I can find that setting directive in Scrapy? Is this what you are referring to 1. I read about directives from the Sphinx Docs.