Add side nav link to Serverless Functions Jamstack Explorers course

[rstavchansky]

Summary

This issue is to suggest adding a link from this site to this Jamstack Explorers mission:
https://explorers.netlify.com/learn/up-and-running-with-serverless-functions

[rstavchansky]

@philhawksworth Heads up on this request

[philhawksworth]

Oooh. Great idea.