Update docs.json

Move build rules to new project structure section.

Author: Amanda Butler

docs.json

@@ -664,6 +664,18 @@
                     }
                 ]
             },
+            {
+                "title": "Project structure",
+                "intro": {
+                    "type": "markdown",
+                    "url": "https://github.com/ARMmbed/mbed-os-5-docs/blob/development/docs/reference/project-structure.md"
+                },
+                "sources": [{
+                        "type": "markdown",
+                        "url": "https://github.com/ARMmbed/mbed-os-5-docs/blob/development/docs/reference/build_rules.md"
+                            }
+                        ]
+            },
             {
                 "title": "Configuration",
                 "intro": {
@@ -941,15 +953,6 @@
                         "url": "https://github.com/ARMmbed/mbed-os-5-docs/blob/development/docs/tools/fastmodels/fastmodels.md"
                     }
                 ]
-            },
-            {
-                "title": "Configuring targets",
-                "sources": [
-                    {
-                        "type": "markdown",
-                        "url": "https://github.com/ARMmbed/mbed-os-5-docs/blob/development/docs/reference/build_rules.md"
-                    }
-                ]
             }
         ],
         "tags": [