Include example of map directive on the Lists page

[bivens-dev]

Ran into a situation today where I was trying to figure out how to work with rendering values in a Set by looking at https://lit.dev/docs/templates/lists/ but was not able to find the answer I was ultimately looking for which was the map directive

By including additional examples on the Working with Lists page to include this it will hopefully help others avoid running into the same issue.

[justinfagnani]

Great point!