Refactor protected branches documentation

app/views/projects/protected_branches/index.html.haml

doc/user/project/img/project_settings_list.png

doc/user/project/img/protected_branches_choose_branch.png

doc/user/project/img/protected_branches_devs_can_push.png

doc/user/project/img/protected_branches_error_ui.png

doc/user/project/img/protected_branches_list.png

doc/user/project/img/protected_branches_matches.png

doc/user/project/protected_branches.md

doc/workflow/README.md

doc/workflow/protected_branches.md

doc/workflow/protected_branches/protected_branches1.png

doc/workflow/protected_branches/protected_branches2.png

doc/workflow/protected_branches/protected_branches3.png