[docs] Provide a way for local users to do the Try LitElement tutorial

[ghost]

Currently, the Try LitElement tutorial is only available as StackBlitz examples.

Provide a way for people to do the tutorial and work locally. E.g. put the Try samples on GitHub and provide instructions for doing the tutorial with Polymer CLI

[nicolejadeyee]

@e111077 flagging this just in case you want to address this in your tutorial work.

[justinfagnani]

We're tracking docs issues in the https://github.com/lit/lit.dev repo now.