Reduce size of images from 25MB to 13MB using pngquant

doc/administration/img/custom_hooks_error_msg.png

...istration/img/high_availability/active-active-diagram.png

...stration/img/high_availability/active-passive-diagram.png

doc/administration/img/housekeeping_settings.png

doc/administration/img/raketasks/check_repos_output.png

doc/administration/img/repository_storages_admin_ui.png

...monitoring/performance/img/grafana_dashboard_dropdown.png

...n/monitoring/performance/img/grafana_dashboard_import.png

...ing/performance/img/grafana_data_source_configuration.png

.../monitoring/performance/img/grafana_data_source_empty.png

...stration/monitoring/performance/img/grafana_save_icon.png

...performance/img/metrics_gitlab_configuration_settings.png

...ion/monitoring/performance/img/request_profile_result.png

...on/monitoring/performance/img/request_profiling_token.png

doc/administration/operations/img/sidekiq_job_throttling.png

doc/ci/img/builds_tab.png

doc/ci/img/deployments_view.png

doc/ci/img/environments_available_staging.png

doc/ci/img/environments_dynamic_groups.png

doc/ci/img/environments_link_url.png