Merge branch 'cross-link-instrumentation' into 'master'

Link to GitLab Performance Monitoring

See merge request gitlab-org/gitlab-ce!31947

doc/development/instrumentation.md

 # Instrumenting Ruby Code
 
-GitLab Performance Monitoring allows instrumenting of both methods and custom
+[GitLab Performance Monitoring](../administration/monitoring/performance/index.md) allows instrumenting of both methods and custom
 blocks of Ruby code. Method instrumentation is the primary form of
 instrumentation with block-based instrumentation only being used when we want to
 drill down to specific regions of code within a method.