Add bare metal book, USB arch page to docs.json

Add bare metal book with bare metal API list and placeholder intro page, and add USB architecture page.

Author: Amanda Butler

docs.json

@@ -107,6 +107,26 @@
             }
         ]
     },
+    {
+        "title": "Mbed OS bare metal",
+        "description": "The Mbed OS bare metal profile",
+        "slug": "introduction",
+        "type": "markdown",
+        "chapters": [{
+                "title": "Introduction",
+                "intro": {
+                    "path": "docs/introduction/introduction.md"
+                },
+                "sources": []
+            },
+            {
+                "title": "Bare metal APIs",
+                "sources": [{
+                    "path": "docs/baremetal_api.md"
+                }]
+            }
+        ]
+    },
     {
         "title": "APIs",
         "description": "The APIs of Mbed OS: platform, drivers, RTOS, connectivity, security and storage",
@@ -663,7 +683,9 @@
                             {
                                 "path": "docs/reference/technology/storage.md"
                             },
-
+                            {
+                                "path": "docs/reference/technology/USB.md"
+                            },
                             {
                                 "path": "docs/reference/runtime/Memory.md"
                             },