[SPARK-33017][PYTHON][DOCS][FOLLOW-UP] Add getCheckpointDir into API documentation
[SPARK-33017][PYTHON][DOCS][FOLLOW-UP] Add getCheckpointDir into API documentation

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

This is a followup of https://github.com/apache/spark/pull/29918. We should add it into the documentation as well.

### Why are the changes needed?

To show users new APIs.

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

Yes, `SparkContext.getCheckpointDir` will be documented.

### How was this patch tested?

Manually built the PySpark documentation:

```bash
cd python/docs
make clean html
cd build/html
open index.html
```

Closes #29960 from HyukjinKwon/SPARK-33017.

Authored-by: HyukjinKwon <2680965ad5aae29275c8fb10150cb10f5554e78b@apache.org>
Signed-off-by: HyukjinKwon <2680965ad5aae29275c8fb10150cb10f5554e78b@apache.org>

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