[SPARK-30574][DOC] Document GROUP BY Clause of SELECT statement in SQL Reference
[SPARK-30574][DOC] Document GROUP BY Clause of SELECT statement in SQL Reference

### What changes were proposed in this pull request?
Document GROUP BY clause of SELECT statement in SQL Reference Guide.

### Why are the changes needed?
Currently Spark lacks documentation on the supported SQL constructs causing
confusion among users who sometimes have to look at the code to understand the
usage. This is aimed at addressing this issue.

### Does this PR introduce any user-facing change?
Yes.

**Before:**
There was no documentation for this.

**After.**
<img width="1093" alt="Screen Shot 2020-01-19 at 5 11 12 PM" src="https://user-images.githubusercontent.com/14225158/72692222-7bf51a00-3adf-11ea-8851-1d313b49020e.png">
<img width="1040" alt="Screen Shot 2020-01-19 at 5 11 32 PM" src="https://user-images.githubusercontent.com/14225158/72692235-90d1ad80-3adf-11ea-947d-df9ab5051069.png">
<img width="1040" alt="Screen Shot 2020-01-19 at 5 11 49 PM" src="https://user-images.githubusercontent.com/14225158/72692257-a8109b00-3adf-11ea-98e8-40742be2ce1a.png">
<img width="1040" alt="Screen Shot 2020-01-19 at 5 12 05 PM" src="https://user-images.githubusercontent.com/14225158/72692372-5d435300-3ae0-11ea-8832-55d9a0426478.png">
<img width="1040" alt="Screen Shot 2020-01-19 at 5 12 31 PM" src="https://user-images.githubusercontent.com/14225158/72692386-69c7ab80-3ae0-11ea-92e4-f1daab6ff897.png">
<img width="960" alt="Screen Shot 2020-01-19 at 5 26 38 PM" src="https://user-images.githubusercontent.com/14225158/72692460-e9ee1100-3ae0-11ea-909e-18e0f90476d9.png">

### How was this patch tested?
Tested using jykyll build --serve

Closes #27283 from dilipbiswal/sql-ref-select-groupby.

Authored-by: Dilip Biswal <1849a9a0cb02515c3e59cb65a673e821c0e87609@gmail.com>
Signed-off-by: Sean Owen <d519fa33a62684a61ab04b24d26f83452d51b34c@gmail.com>

vinodkc/spark,HyukjinKwon/spark,shuangshuangwang/spark,kevinyu98/spark,maropu/spark,ConeyLiu/spark,dbtsai/spark,taroplus/spark,ueshin/apache-spark,zuotingbing/spark,darionyaphet/spark,holdenk/spark,rednaxelafx/apache-spark,zero323/spark,kevinyu98/spark,ptkool/spark,nchammas/spark,chuckchen/spark,srowen/spark,zzcclp/spark,shuangshuangwang/spark,maropu/spark,xuanyuanking/spark,hvanhovell/spark,dbtsai/spark,vinodkc/spark,dbtsai/spark,kevinyu98/spark,spark-test/spark,maropu/spark,gengliangwang/spark,milliman/spark,ptkool/spark,WeichenXu123/spark,shuangshuangwang/spark,skonto/spark,wzhfy/spark,ueshin/apache-spark,kevinyu98/spark,skonto/spark,witgo/spark,HyukjinKwon/spark,WeichenXu123/spark,wzhfy/spark,ptkool/spark,rednaxelafx/apache-spark,goldmedal/spark,goldmedal/spark,srowen/spark,jiangxb1987/spark,JoshRosen/spark,wzhfy/spark,holdenk/spark,hvanhovell/spark,maropu/spark,dbtsai/spark,apache/spark,shuangshuangwang/spark,gengliangwang/spark,JoshRosen/spark,mahak/spark,chuckchen/spark,spark-test/spark,nchammas/spark,JoshRosen/spark,chuckchen/spark,zero323/spark,hvanhovell/spark,shaneknapp/spark,darionyaphet/spark,xuanyuanking/spark,rednaxelafx/apache-spark,milliman/spark,dongjoon-hyun/spark,jiangxb1987/spark,chuckchen/spark,dongjoon-hyun/spark,witgo/spark,wangyum/spark,gengliangwang/spark,mahak/spark,HyukjinKwon/spark,goldmedal/spark,skonto/spark,zzcclp/spark,ConeyLiu/spark,nchammas/spark,jiangxb1987/spark,gengliangwang/spark,dbtsai/spark,mahak/spark,apache/spark,WeichenXu123/spark,wangyum/spark,srowen/spark,taroplus/spark,taroplus/spark,skonto/spark,nchammas/spark,WeichenXu123/spark,wzhfy/spark,zuotingbing/spark,taroplus/spark,goldmedal/spark,apache/spark,dongjoon-hyun/spark,zero323/spark,holdenk/spark,srowen/spark,BryanCutler/spark,wzhfy/spark,witgo/spark,taroplus/spark,hvanhovell/spark,xuanyuanking/spark,wangyum/spark,wangmiao1981/spark,chuckchen/spark,apache/spark,hvanhovell/spark,nchammas/spark,xuanyuanking/spark,shuangshuangwang/spark,ueshin/apache-spark,wangmiao1981/spark,witgo/spark,witgo/spark,milliman/spark,wangmiao1981/spark,shaneknapp/spark,kevinyu98/spark,jiangxb1987/spark,BryanCutler/spark,cloud-fan/spark,goldmedal/spark,HyukjinKwon/spark,JoshRosen/spark,wangyum/spark,mahak/spark,zero323/spark,darionyaphet/spark,srowen/spark,gengliangwang/spark,spark-test/spark,ConeyLiu/spark,shuangshuangwang/spark,gengliangwang/spark,JoshRosen/spark,wangmiao1981/spark,apache/spark,chuckchen/spark,milliman/spark,maropu/spark,zuotingbing/spark,milliman/spark,spark-test/spark,zuotingbing/spark,spark-test/spark,kevinyu98/spark,zero323/spark,taroplus/spark,srowen/spark,wangmiao1981/spark,chuckchen/spark,dongjoon-hyun/spark,ConeyLiu/spark,BryanCutler/spark,xuanyuanking/spark,taroplus/spark,vinodkc/spark,mahak/spark,dongjoon-hyun/spark,darionyaphet/spark,rednaxelafx/apache-spark,rednaxelafx/apache-spark,vinodkc/spark,ptkool/spark,ueshin/apache-spark,shaneknapp/spark,dongjoon-hyun/spark,goldmedal/spark,WeichenXu123/spark,holdenk/spark,apache/spark,spark-test/spark,wangyum/spark,skonto/spark,mahak/spark,dbtsai/spark,ueshin/apache-spark,zzcclp/spark,HyukjinKwon/spark,vinodkc/spark,HyukjinKwon/spark,xuanyuanking/spark,shaneknapp/spark,ConeyLiu/spark,skonto/spark,ConeyLiu/spark,vinodkc/spark,zuotingbing/spark,holdenk/spark,JoshRosen/spark,JoshRosen/spark,wzhfy/spark,holdenk/spark,wangyum/spark,spark-test/spark,BryanCutler/spark,mahak/spark,cloud-fan/spark,WeichenXu123/spark,zero323/spark,zzcclp/spark,ptkool/spark,milliman/spark,zzcclp/spark,BryanCutler/spark,jiangxb1987/spark,cloud-fan/spark,shaneknapp/spark,skonto/spark,shaneknapp/spark,goldmedal/spark,xuanyuanking/spark,cloud-fan/spark,maropu/spark,wangmiao1981/spark,darionyaphet/spark,zuotingbing/spark,gengliangwang/spark,hvanhovell/spark,matthewfranglen/spark,rednaxelafx/apache-spark,jiangxb1987/spark,cloud-fan/spark,zzcclp/spark,WeichenXu123/spark,shaneknapp/spark,hvanhovell/spark,nchammas/spark,witgo/spark,zzcclp/spark,shuangshuangwang/spark,ptkool/spark,ueshin/apache-spark,jiangxb1987/spark,wzhfy/spark,HyukjinKwon/spark,nchammas/spark,milliman/spark,ueshin/apache-spark,witgo/spark,cloud-fan/spark,darionyaphet/spark,holdenk/spark,rednaxelafx/apache-spark,vinodkc/spark,BryanCutler/spark,wangmiao1981/spark,apache/spark,zuotingbing/spark,maropu/spark,BryanCutler/spark,dbtsai/spark,darionyaphet/spark,ConeyLiu/spark,srowen/spark,ptkool/spark,wangyum/spark,kevinyu98/spark,cloud-fan/spark,dongjoon-hyun/spark,zero323/spark