Restructure documentation

doc/administration/customization/branded_login_page.md

...istration/customization/branded_login_page/appearance.png

...ation/customization/branded_login_page/custom_sign_in.png

...n/customization/branded_login_page/default_login_page.png

doc/administration/customization/issue_closing.md

doc/administration/customization/libravatar.md

doc/administration/customization/welcome_message.md

doc/administration/downgrade_ee_to_ce/README.md

doc/administration/hooks/custom_hooks.md

doc/administration/incoming_email/README.md

doc/administration/incoming_email/postfix.md

doc/administration/integration/README.md

doc/administration/integration/akismet.md

doc/administration/integration/auth0.md

doc/administration/integration/azure.md

doc/administration/integration/bitbucket.md

doc/administration/integration/cas.md

doc/administration/integration/crowd.md

doc/administration/integration/external-issue-tracker.md

doc/administration/integration/facebook.md