Add link to collaborate on slack

[oskoss]

Hi Y'all! I'd like to use your docs theme for a documentation project. I have gotten most of it setup, but am wondering if y'all could point me in the right direction of where to start if I wanted to place a link to a slack team within the side bar above the edit on github link?

Thanks!

[coldice]

This item is defined here: https://github.com/apollographql/gatsby-theme-apollo/blob/master/packages/gatsby-theme-apollo-docs/src/components/page-content.js#L186.
You can use shadowing to extend it.