User Guide: Remove unused file section_dashboard_manage_instances.xml

section_dashboard_manage_instances.xml is unused.
The content of section_dashboard_manage_instances.xml is completely
in section_dashboard_launch_instances.xml.

Change-Id: I428857ed65c179881bbf623d6f5abfb36e12b187
Merge "User Guide: Remove unused file section_dashboard_manage_instances.xml"