Migration of Drupal 7 theme Documentation

Problem/Motivation

The Site Building Guide is being migrated to the Drupal 7 Wiki.
https://www.drupal.org/documentation/build
https://www.drupal.org/docs/7/themes

This issue is to document that the documentation has been removed.

Remaining tasks

Please update the documentation links on the project page so they point to the new documentation:
https://www.drupal.org/docs/7/themes