xsl:for-each-group tutorial

[ebeshero]

@spadafour will draft a tutorial for newtfire on xsl:for-each-group, working from our posts on the DHClass-Hub, aiming for next Wed. 11/11, to help people use this in projects.

The starter explainForEachGroup.html page we drafted is here in the newtFire-webDev git:
https://github.com/ebeshero/newtFire-webDev/blob/master/dh-course/explainForEachGroup.html

Thanks, Rob! If you have questions or problems as you're working on this, post them here!