Link to style guide

[rgaiacs]

Related to https://github.com/carpentries/handbook/issues/436

[bencomp]

I noticed that the link to the style guide in the section Style guide should be updated to https://docs.carpentries.org/topic_folders/communications/resources/style-guide.html.

It's clear that there is discussion on where the official style guide should be. I would indeed prefer having one style guide that I need to check as a maintainer/lesson contributor. While that discussion is ongoing, this issue looked appropriate to mention this broken link.