document `<output>` tags for layout based templating

[thescientist13]

Summary

In ProjectEvergreen/greenwood#1600, we introduced a (breaking) change to how content based layout templating was done, now using the <output> tag

Instead of using

<content-outlet></content-outlet>
<page-outlet></page-outlet>

It is now

<output for="content"></outlet>
<output for="page"></outlet>

Additional Details

Will need to update all related docs to reflect this new API and the getting started guide

Greenwood Issue

ProjectEvergreen/greenwood#1476