Merge branch 'docs-labels-cleanup' into 'master'

doc/user/project/img/labels_default_v12_1.png

doc/user/project/img/labels_epic_sidebar_v12_1.png

doc/user/project/img/labels_generate_default_v12_1.png

doc/user/project/img/labels_group_issues_v12_1.png

doc/user/project/img/labels_key_value_v12_1.png

doc/user/project/img/labels_project_list_search.png

doc/user/project/img/labels_promotion_v12_1.png

doc/user/project/img/labels_sidebar.png

doc/user/project/img/labels_sidebar_assign.png

doc/user/project/issue_board.md

doc/user/project/labels.md

doc/user/search/index.md