Refactor and restructure integration documentation

We provide many options to auth in GitLab and the docs are scattered in two places: doc/integration/ and doc/administration/auth/. Since these are admin facing docs, they should be under the latter location.

Plan:

• Move doc/integration/*.md to doc/administration/auth/*.md except for the README.md
• Leave doc/integration/README.md and list all integrations (?)
• List all apps that integrate with us like https://docs.codeclimate.com/docs/gitlab. See https://about.gitlab.com/applications
• Move oauth_provider.md somewhere in user/
• Move recaptcha.md to user/admin_area/settings/
• Move akismet.md to user/admin_area/settings/
• Move external-issue-tracker.md to user/project/services (after https://gitlab.com/gitlab-org/gitlab-ce/issues/21741)
• Move gmail_action_buttons_for_gitlab.md to somewhere in user/
• Fill in gitlab.yml.example with all providers and links to new doc location
• Submit MR to EE