update site dox - windows prefer msvs-ninja and add vulkan

site/user/special/vulkan.md

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+Building the Vulkan backend
+===========================
+
+Introduction
+------------
+
+The Vulkan backend is experimental and not built by default. It currently replaces the 'gpu' config in the dm tool rather than running in addition to OpenGL. Vulkan has been built on Windows and Linux.
+
+Details
+-------
+
+Add `skia_vulkan=1` to your `GYP_DEFINES` environment variable. Ensure cmake is installed and in your path. On Windows install the prebuilt cmake from https://cmake.org/download/. The Vulkan SDK must also be installed.
+
+On Windows the 'msvs' gyp generator will not work. Use msvs-ninja for MSVS integration.
+
+Run:
+
+    python gyp_skia
+
+and build Skia as usual.