Add links to documentation to index metadata

[ZacharyKlein]

We should provide at least two links to documentation in the default JSON index response:

• Grails Documentation (http://docs.grails.org/latest/guide/single.html)
• Profile Documentation (https://grails-profiles.github.io/rest-api/latest/guide/index.html)

E.g.:

{  
   "message":"Welcome to Grails!",
   "environment":"development",
   "appversion":"0.1",
   "grailsversion":"3.3.2",
   "appprofile":"rest-api",
   "documentation": "http://docs.grails.org/latest/guide/single.html",
   "profiledocumentation": "https://grails-profiles.github.io/rest-api/latest/guide/index.html",