Issue #2854576 by nonprofit, meenakshig, markconroy, namitasaxena,...

Issue #2854576 by nonprofit, meenakshig, markconroy, namitasaxena, vsujeetkumar, fabienly, andrewtur, borisson_, xjm: Discourage use of Stark as a base theme (and possibly mark it internal or hidden)

core/themes/stark/stark.info.yml

 name: Stark
 type: theme
-description: 'An intentionally plain theme with no styling to demonstrate default Drupal’s HTML and CSS. Learn how to build a custom theme from Stark in the <a href="https://www.drupal.org/docs/8/theming">Theming Guide</a>.'
+description: 'An intentionally plain theme to demonstrate and test default HTML and CSS. Stark should not be used as a base theme. Instead, build a custom theme using the <a href="https://www.drupal.org/docs/develop/theming-drupal">Theming Guide</a> as a reference.'
 package: Core
 version: VERSION
 base theme: false