Added DocFX documentation

docs/api/index.md

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+# Library reference
+
+**Select your platform:**
+
+- [.NET Standard API Reference](netstd/index.html)
+
+- [.NET Framework API Reference](netfx/index.html)
+
+- [.NET Core API Reference](netcore/index.html)
+
+- [UWP API Reference](uwp/index.html)
+
+- [Android API Reference](android/index.html)
+
+- [iOS API Reference](ios/index.html)
+
+### Object model diagram:
+
+[Click here](omd.html) to see the full object model diagram
+
+### Platform specific devices:
+
+Note: The library reference is identical on each platform, except some platforms have platform-specific additional NMEA devices to simplify usage on those platforms. While they may be named the same, they might be have slightly different or take different platform-specific arguments.
+See the menu in the "[Getting Started](../concepts/index.html)" section for more info on these.