## What changes were proposed in this pull request?

Support using latex in scaladoc by adding MathJax javascript to the js template.

## How was this patch tested?

Generated scaladoc.  Preview:

- LogisticGradient: [before](https://spark.apache.org/docs/2.0.0/api/scala/index.html#org.apache.spark.mllib.optimization.LogisticGradient) and [after](https://sparkdocs.lins05.pw/spark-16822/api/scala/index.html#org.apache.spark.mllib.optimization.LogisticGradient)

- MinMaxScaler: [before](https://spark.apache.org/docs/2.0.0/api/scala/index.html#org.apache.spark.ml.feature.MinMaxScaler) and [after](https://sparkdocs.lins05.pw/spark-16822/api/scala/index.html#org.apache.spark.ml.feature.MinMaxScaler)

Author: Shuai Lin <linshuai2012@gmail.com>

Closes #14438 from lins05/spark-16822-support-latex-in-scaladoc.