Add image for clearing the cache docs

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/4642
5 jobs for docs/cache-clear-image in 3 minutes and 22 seconds (queued for 6 seconds)
Status Job ID Name Coverage
  Build
manual #58855070
gitlab-org allowed to fail manual
package-and-qa
manual #58855072
gitlab-org allowed to fail manual
review-docs-deploy
 
  Test
passed #58855074
gitlab-org
docs lint

00:00:25

passed #58855075
gitlab-org
ee_compat_check

00:03:22

 
  Post Cleanup
manual #58855076
gitlab-org allowed to fail manual
review-docs-cleanup