### What changes were proposed in this pull request? Add a few main pages ### Why are the changes needed? To make SQL Reference complete. ### Does this PR introduce any user-facing change? Yes ![image](https://user-images.githubusercontent.com/13592258/73563358-f859f800-4411-11ea-8bd9-27d4db784957.png) ![image](https://user-images.githubusercontent.com/13592258/73530590-a55e5180-43cd-11ea-81b9-0192ff990b96.png) ![image](https://user-images.githubusercontent.com/13592258/73530629-b909b800-43cd-11ea-91a9-cfc71e213c7a.png) ![image](https://user-images.githubusercontent.com/13592258/73530812-0be36f80-43ce-11ea-9151-efa4ab7f2105.png) ![image](https://user-images.githubusercontent.com/13592258/73530908-3e8d6800-43ce-11ea-9943-10f2bd2bb408.png) ![image](https://user-images.githubusercontent.com/13592258/73530916-451bdf80-43ce-11ea-83c2-c7a9b063add7.png) ![image](https://user-images.githubusercontent.com/13592258/73530927-4baa5700-43ce-11ea-963c-951c8820ff54.png) ![image](https://user-images.githubusercontent.com/13592258/73530963-5cf36380-43ce-11ea-8cb1-6064ba2992f3.png) ### How was this patch tested? Manually build and check Closes #27416 from huaxingao/spark-doc. Authored-by: Huaxin Gao <huaxing@us.ibm.com> Signed-off-by: Sean Owen <srowen@gmail.com>