Add some description and a link to the identity matrix

Author: maxim-masterov

docs/test-suite/dashboard.md

@@ -1,5 +1,10 @@
 # Dashboard for the test suite
 
-[https://dashboard.eessi.io](https://dashboard.eessi.io)
+## Performance dashboard
+This dashboard offers an intuitive and straightforward way to visualize test results generated by the
+EESSI ReFrame test suite: [https://dashboard.eessi.io](https://dashboard.eessi.io)
+
+## Identity matrix
+The identity matrix presents the success and failure rates of executed tests: [https://dashboard.eessi.io/module-matrix/](https://dashboard.eessi.io/module-matrix/)
 
 *(more info soon)*