Create named z-index utilities

[alizedebray]

We will soon introduce z-index utilities (#5741).

However, these utilities currently only allow setting an element a few steps above or below its default index.

We also use named z-index values for elements that appear above the page content, such as headers, modals, dropdowns, etc.

Goal

• Clean up the named z-indexes: ensure all used z-index values are registered in this map and that the numbers are coherent
• Create a Sass map containing all named z-index values
• Generate z-index utilities from the utility API using this map
• Document the named z-index classes on the z-index utility documentation page