[SPARK-21069][SS][DOCS] Add rate source to programming guide.
[SPARK-21069][SS][DOCS] Add rate source to programming guide.

## What changes were proposed in this pull request?

SPARK-20979 added a new structured streaming source: Rate source. This patch adds the corresponding documentation to programming guide.

## How was this patch tested?

Tested by running jekyll locally.

Author: Prashant Sharma <prashant@apache.org>
Author: Prashant Sharma <prashsh1@in.ibm.com>

Closes #18562 from ScrapCodes/spark-21069/rate-source-docs.

yanboliang/spark,rednaxelafx/apache-spark,mahak/spark,milliman/spark,nchammas/spark,adrian-ionescu/apache-spark,guoxiaolongzte/spark,goldmedal/spark,alunarbeach/spark,minixalpha/spark,caneGuy/spark,xuanyuanking/spark,zuotingbing/spark,UndeadBaneGitHub/spark,cloud-fan/spark,HyukjinKwon/spark,pgandhi999/spark,bravo-zhang/spark,ueshin/apache-spark,bravo-zhang/spark,BryanCutler/spark,ddna1021/spark,esi-mineset/spark,narahari92/spark,rednaxelafx/apache-spark,pgandhi999/spark,jkbradley/spark,byakuinss/spark,goldmedal/spark,wzhfy/spark,ajaysaini725/spark,brad-kaiser/spark,jiangxb1987/spark,nilsgrabbert/spark,bravo-zhang/spark,debugger87/spark,saltstar/spark,WindCanDie/spark,mahak/spark,zuotingbing/spark,gengliangwang/spark,ericvandenbergfb/spark,wangyum/spark,WindCanDie/spark,bdrillard/spark,ioana-delaney/spark,mahak/spark,ptkool/spark,Aegeaner/spark,darionyaphet/spark,aray/spark,facaiy/spark,wzhfy/spark,janewangfb/spark,ConeyLiu/spark,joseph-torres/spark,shaneknapp/spark,apache/spark,eyalfa/spark,holdenk/spark,rezasafi/spark,maropu/spark,skonto/spark,shubhamchopra/spark,pgandhi999/spark,cin/spark,kiszk/spark,witgo/spark,aokolnychyi/spark,holdenk/spark,hhbyyh/spark,aokolnychyi/spark,milliman/spark,HyukjinKwon/spark,aray/spark,wangmiao1981/spark,shuangshuangwang/spark,JoshRosen/spark,mdespriee/spark,vinodkc/spark,rikima/spark,dbtsai/spark,joseph-torres/spark,cloud-fan/spark,stanzhai/spark,tejasapatil/spark,dongjoon-hyun/spark,wzhfy/spark,apache/spark,jiangxb1987/spark,hhbyyh/spark,aosagie/spark,tejasapatil/spark,stanzhai/spark,icexelloss/spark,rekhajoshm/spark,techaddict/spark,debugger87/spark,dongjoon-hyun/spark,narahari92/spark,goldmedal/spark,holdenk/spark,hvanhovell/spark,szhem/spark,gengliangwang/spark,caneGuy/spark,WeichenXu123/spark,kiszk/spark,BryanCutler/spark,srowen/spark,zero323/spark,nchammas/spark,janewangfb/spark,lxsmnv/spark,tengpeng/spark,facaiy/spark,chuckchen/spark,shuangshuangwang/spark,jkbradley/spark,saltstar/spark,aokolnychyi/spark,kevinyu98/spark,LantaoJin/spark,witgo/spark,goldmedal/spark,gengliangwang/spark,shuangshuangwang/spark,HyukjinKwon/spark,ajaysaini725/spark,sahilTakiar/spark,nlalevee/spark,minixalpha/spark,witgo/spark,nchammas/spark,rekhajoshm/spark,xuanyuanking/spark,yanboliang/spark,ConeyLiu/spark,ericvandenbergfb/spark,someorz/spark,rezasafi/spark,LantaoJin/spark,janewangfb/spark,chuckchen/spark,cloud-fan/spark,JoshRosen/spark,techaddict/spark,aosagie/spark,icexelloss/spark,darionyaphet/spark,actuaryzhang/spark,tengpeng/spark,techaddict/spark,zero323/spark,sureshthalamati/spark,eyalfa/spark,ConeyLiu/spark,minixalpha/spark,dbtsai/spark,zuotingbing/spark,nlalevee/spark,rednaxelafx/apache-spark,ddna1021/spark,guoxiaolongzte/spark,tejasapatil/spark,1haodian/spark,nlalevee/spark,kiszk/spark,yanboliang/spark,saltstar/spark,rednaxelafx/apache-spark,facaiy/spark,ConeyLiu/spark,someorz/spark,someorz/spark,zhouyejoe/spark,narahari92/spark,ahnqirage/spark,rikima/spark,adrian-ionescu/apache-spark,HyukjinKwon/spark,guoxiaolongzte/spark,esi-mineset/spark,lvdongr/spark,ron8hu/spark,tejasapatil/spark,adrian-ionescu/apache-spark,rikima/spark,ajaysaini725/spark,BryanCutler/spark,1haodian/spark,WeichenXu123/spark,xflin/spark,maropu/spark,LantaoJin/spark,hhbyyh/spark,lxsmnv/spark,jkbradley/spark,szhem/spark,ConeyLiu/spark,witgo/spark,bravo-zhang/spark,witgo/spark,brad-kaiser/spark,brad-kaiser/spark,ptkool/spark,nlalevee/spark,LantaoJin/spark,SHASHANKB/spark,tejasapatil/spark,ahnqirage/spark,hhbyyh/spark,WindCanDie/spark,hvanhovell/spark,alunarbeach/spark,dongjoon-hyun/spark,mahak/spark,caneGuy/spark,zero323/spark,yanboliang/spark,nchammas/spark,dongjoon-hyun/spark,stanzhai/spark,lvdongr/spark,byakuinss/spark,mike0sv/spark,darionyaphet/spark,eyalfa/spark,kevinyu98/spark,spark-test/spark,adrian-ionescu/apache-spark,highfei2011/spark,ptkool/spark,debugger87/spark,minixalpha/spark,facaiy/spark,sureshthalamati/spark,Aegeaner/spark,byakuinss/spark,matthewfranglen/spark,hhbyyh/spark,bdrillard/spark,icexelloss/spark,shubhamchopra/spark,mike0sv/spark,cin/spark,milliman/spark,gengliangwang/spark,UndeadBaneGitHub/spark,shubhamchopra/spark,esi-mineset/spark,highfei2011/spark,vinodkc/spark,shaneknapp/spark,alunarbeach/spark,narahari92/spark,dbtsai/spark,darionyaphet/spark,saltstar/spark,wangyum/spark,caneGuy/spark,cloud-fan/spark,JoshRosen/spark,tengpeng/spark,ioana-delaney/spark,zzcclp/spark,wangmiao1981/spark,joseph-torres/spark,janewangfb/spark,nchammas/spark,shaneknapp/spark,wzhfy/spark,debugger87/spark,xuanyuanking/spark,janewangfb/spark,debugger87/spark,taroplus/spark,caneGuy/spark,pgandhi999/spark,wangyum/spark,nchammas/spark,joseph-torres/spark,mdespriee/spark,mahak/spark,shuangshuangwang/spark,vinodkc/spark,eyalfa/spark,szhem/spark,highfei2011/spark,wzhfy/spark,darionyaphet/spark,chuckchen/spark,skonto/spark,apache/spark,pgandhi999/spark,zzcclp/spark,LantaoJin/spark,mike0sv/spark,markhamstra/spark,byakuinss/spark,lvdongr/spark,ahnqirage/spark,lvdongr/spark,rikima/spark,akopich/spark,szhem/spark,bdrilla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