[SPARK-30823][PYTHON][DOCS] Set `%PYTHONPATH%` when building PySpark documentation on Windows
[SPARK-30823][PYTHON][DOCS] Set `%PYTHONPATH%` when building PySpark documentation on Windows

This commit is published into the public domain.

### What changes were proposed in this pull request?
In analogy to `python/docs/Makefile`, which has
> export PYTHONPATH=$(realpath ..):$(realpath ../lib/py4j-0.10.8.1-src.zip)

on line 10, this PR adds
> set PYTHONPATH=..;..\lib\py4j-0.10.8.1-src.zip

to `make2.bat`.

Since there is no `realpath` in default installations of Windows, I left the relative paths unresolved. Per the instructions on how to build docs, `make.bat` is supposed to be run from `python/docs` as the working directory, so this should probably not cause issues (`%BUILDDIR%` is a relative path as well.)

### Why are the changes needed?
When building the PySpark documentation on Windows, by changing directory to `python/docs` and running `make.bat` (which runs `make2.bat`), the majority of the documentation may not be built if pyspark is not in the default `%PYTHONPATH%`. Sphinx then reports that `pyspark` (and possibly dependencies) cannot be imported.

If `pyspark` is in the default `%PYTHONPATH%`, I suppose it is that version of `pyspark` – as opposed to the version found above the `python/docs` directory – that is considered when building the documentation, which may result in documentation that does not correspond to the development version one is trying to build.

### Does this PR introduce any user-facing change?
No.

### How was this patch tested?
Manual tests on my Windows 10 machine. Additional tests with other environments very welcome!

Closes #27569 from DavidToneian/SPARK-30823.

Authored-by: David Toneian <aa743a0aaec8f7d7a1f01442503957f4d7a2d634@toneian.com>
Signed-off-by: HyukjinKwon <2680965ad5aae29275c8fb10150cb10f5554e78b@apache.org>

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