[SPARK-34022][DOCS] Support latest mkdocs in SQL built-in function docs
[SPARK-34022][DOCS] Support latest mkdocs in SQL built-in function docs

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

This PR adds the support of the latest mkdocs, and makes the sidebar properly show. It works in lower versions too.

Before:

![Screen Shot 2021-01-06 at 5 11 56 PM](https://user-images.githubusercontent.com/6477701/103745131-4e7fe400-5042-11eb-9c09-84f9f95e9fb9.png)

After:

![Screen Shot 2021-01-06 at 5 10 53 PM](https://user-images.githubusercontent.com/6477701/103745139-5049a780-5042-11eb-8ded-30b6f7ef48aa.png)

### Why are the changes needed?

This is a regression in the documentation.

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

Technically no. It's not related yet. It fixes the list on the sidebar appears properly.

### How was this patch tested?

Manually built the docs via `./sql/create-docs.sh` and `open ./sql/site/index.html`

Closes #31061 from HyukjinKwon/SPARK-34022.

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

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