Documentation for Admin theme

Problem/Motivation

Adding an issue for the 'Documentation' item on the parent issue. This is a stable blocker

Steps to reproduce

Proposed resolution

Review existing documentation, https://www.drupal.org/docs/core-modules-and-themes/core-themes/default-...

Remaining tasks

What, if anything, needs to changed or added?

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet