Link features to new docs/pages

Update https://about.gitlab.com/features/:

• Service Desk - link to https://about.gitlab.com/features/service-desk/
• Powerful Issue Tracker - link to https://docs.gitlab.com/ee/user/project/issues/
• Work-in-Progress Protection - link to https://docs.gitlab.com/ee/user/project/merge_requests/work_in_progress_merge_requests.html
• Group levels milestones - link to https://docs.gitlab.com/ee/user/project/milestones/index.html#creating-a-group-milestone
• Burndown charts - link to https://docs.gitlab.com/ee/user/project/milestones/burndown_charts.html
• Multiple Assignees - link to https://docs.gitlab.com/ee/user/project/issues/multiple_assignees_for_issues.html
• Multi-Project Pipeline Graphs - link to https://docs.gitlab.com/ee/ci/multi_project_pipeline_graphs.html