[SPARK-33641][SQL][DOC][FOLLOW-UP] Add migration guide for CHAR VARCHAR types
[SPARK-33641][SQL][DOC][FOLLOW-UP] Add migration guide for CHAR VARCHAR types

### What changes were proposed in this pull request?

Add migration guide for CHAR VARCHAR types

### Why are the changes needed?

for migration

### Does this PR introduce _any_ user-facing change?

doc change

### How was this patch tested?

passing ci

Closes #30654 from yaooqinn/SPARK-33641-F.

Authored-by: Kent Yao <bee1182b70536910b4db11bba3bcb19b1c82ecd8@hotmail.com>
Signed-off-by: Wenchen Fan <563f5923eae9a92c8a3f34a26b67fa348a303ed1@databricks.com>

cloud-fan/spark,zero323/spark,srowen/spark,BryanCutler/spark,xuanyuanking/spark,mahak/spark,holdenk/spark,vinodkc/spark,apache/spark,cloud-fan/spark,maropu/spark,JoshRosen/spark,zzcclp/spark,shaneknapp/spark,milliman/spark,milliman/spark,rednaxelafx/apache-spark,jiangxb1987/spark,zero323/spark,wangmiao1981/spark,nchammas/spark,witgo/spark,rednaxelafx/apache-spark,holdenk/spark,HyukjinKwon/spark,wangyum/spark,wangmiao1981/spark,gengliangwang/spark,mahak/spark,nchammas/spark,apache/spark,vinodkc/spark,JoshRosen/spark,ueshin/apache-spark,ueshin/apache-spark,BryanCutler/spark,srowen/spark,hvanhovell/spark,ueshin/apache-spark,hvanhovell/spark,zero323/spark,gengliangwang/spark,jiangxb1987/spark,dongjoon-hyun/spark,nchammas/spark,taroplus/spark,milliman/spark,shaneknapp/spark,wangyum/spark,xuanyuanking/spark,witgo/spark,WeichenXu123/spark,wangmiao1981/spark,xuanyuanking/spark,apache/spark,dongjoon-hyun/spark,witgo/spark,chuckchen/spark,wangyum/spark,vinodkc/spark,shaneknapp/spark,nchammas/spark,srowen/spark,wangmiao1981/spark,mahak/spark,jiangxb1987/spark,zero323/spark,apache/spark,HyukjinKwon/spark,BryanCutler/spark,maropu/spark,hvanhovell/spark,cloud-fan/spark,JoshRosen/spark,JoshRosen/spark,mahak/spark,milliman/spark,nchammas/spark,JoshRosen/spark,cloud-fan/spark,zzcclp/spark,holdenk/spark,WeichenXu123/spark,shaneknapp/spark,HyukjinKwon/spark,dongjoon-hyun/spark,xuanyuanking/spark,srowen/spark,chuckchen/spark,milliman/spark,mahak/spark,taroplus/spark,HyukjinKwon/spark,xuanyuanking/spark,BryanCutler/spark,BryanCutler/spark,HyukjinKwon/spark,WeichenXu123/spark,rednaxelafx/apache-spark,vinodkc/spark,xuanyuanking/spark,HyukjinKwon/spark,nchammas/spark,vinodkc/spark,BryanCutler/spark,HyukjinKwon/spark,hvanhovell/spark,dongjoon-hyun/spark,milliman/spark,wangmiao1981/spark,zero323/spark,gengliangwang/spark,hvanhovell/spark,cloud-fan/spark,zzcclp/spark,mahak/spark,chuckchen/spark,vinodkc/spark,jiangxb1987/spark,hvanhovell/spark,maropu/spark,apache/spark,vinodkc/spark,gengliangwang/spark,wangmiao1981/spark,srowen/spark,wangyum/spark,chuckchen/spark,ueshin/apache-spark,taroplus/spark,maropu/spark,shaneknapp/spark,rednaxelafx/apache-spark,gengliangwang/spark,taroplus/spark,rednaxelafx/apache-spark,witgo/spark,zzcclp/spark,mahak/spark,witgo/spark,WeichenXu123/spark,jiangxb1987/spark,zero323/spark,taroplus/spark,milliman/spark,witgo/spark,wangmiao1981/spark,rednaxelafx/apache-spark,holdenk/spark,cloud-fan/spark,jiangxb1987/spark,zzcclp/spark,cloud-fan/spark,gengliangwang/spark,JoshRosen/spark,maropu/spark,WeichenXu123/spark,srowen/spark,BryanCutler/spark,zzcclp/spark,dongjoon-hyun/spark,holdenk/spark,srowen/spark,wangyum/spark,maropu/spark,chuckchen/spark,ueshin/apache-spark,WeichenXu123/spark,maropu/spark,ueshin/apache-spark,shaneknapp/spark,WeichenXu123/spark,witgo/spark,holdenk/spark,jiangxb1987/spark,taroplus/spark,apache/spark,JoshRosen/spark,holdenk/spark,taroplus/spark,ueshin/apache-spark,nchammas/spark,hvanhovell/spark,chuckchen/spark,chuckchen/spark,wangyum/spark,shaneknapp/spark,zero323/spark,wangyum/spark,xuanyuanking/spark,apache/spark,dongjoon-hyun/spark,dongjoon-hyun/spark,gengliangwang/spark,zzcclp/spark,rednaxelafx/apache-spark