Add-case-study

Added case study page
Added DVSA practical driving test case study under case study page
Renamed metrics page under user-manual to performance-metrics - undo clash of page names
Added developer page as holder
Updated user-manual index and building-a-dashboard index to add links to case studies
Merge pull request #31 from alphagov/Add-case-study

Add-case-study
Link to how we’ve met each need