Link to Documentation Styleguide

I needed to create some documentation for Wiki's, requested here: https://gitlab.zendesk.com/agent/tickets/43291

First wasted a bunch of time trying to look at the directory structure of the current documentation and trying to figure out where things should go.

Then I found the Styleguide.

Hopefully linking to the Styleguide will save the next person some time, and keep improving the quality of our Docs.