Add SPI Doxygen branch to docs.json

Create SPI rendered Doxygen feature branch.

Author: Amanda Butler

docs.json

@@ -1571,6 +1571,15 @@
         "source": "https://github.com/ARMmbed/mbed-os",
         "branch": "feature-hal-spec-sai"
     },
+    {
+        "title": "feature-hal-spec-spi doxygen",
+        "description": "The full mbed OS API documentation in doxygen format",
+        "slug": "feature-hal-spec-spi-doxy",
+        "hidden": true,
+        "type": "doxygen",
+        "source": "https://github.com/ARMmbed/mbed-os",
+        "branch": "feature-hal-spec-spi"
+    }, 
     {
         "title": "feature-hal-spec-watchdog doxygen",
         "description": "The full mbed OS API documentation in doxygen format",