Deprecate and replace the grid system

[geoffrey-eisenbarth]

Deprecate the convoluted grid system. Replace its docs page with a guide on defining grids tersely with @scope. - Missing Roadmap

Blocked: @scope in Firefox. Blocked: attr() for <custom-ident> in Firefox (I think? mdn isn't clear on this)

Possible new syntax from @dz4k

<style>
.grid {
  [id] {
    grid-area: attr(id);
  }
}
</style>

<div class="grid" style="
  grid-template: if(
    media(width < 768px), 'title' 'nav' 'content',
    'title title' 'nav content' / auto 1fr
  )
">
  <h1 id=title>...</h1>
  <nav id=nav>...</nav>
  <div id=content>...</nav>