[SPARK-20506][DOCS] Add HTML links to highlight list in MLlib guide for 2.2
[SPARK-20506][DOCS] Add HTML links to highlight list in MLlib guide for 2.2

Quick follow up to #17996 - forgot to add the HTML links to the relevant sections of the guide in the highlights list.

## How was this patch tested?

Built docs locally and tested links.

Author: Nick Pentreath <6fe12bfdafd6d3c41da5ad6593c53ed449c85282@za.ibm.com>

Closes #18043 from MLnick/SPARK-20506-2.2-migration-guide-2.

(cherry picked from commit be846db48b226de2b0dfb5f87d059eda15ecf7cd)
Signed-off-by: Nick Pentreath <6fe12bfdafd6d3c41da5ad6593c53ed449c85282@za.ibm.com>

apache-spark-on-k8s/spark,publicRoman/spark,publicRoman/spark,jlopezmalla/spark,publicRoman/spark,apache-spark-on-k8s/spark,apache-spark-on-k8s/spark,apache-spark-on-k8s/spark,jlopezmalla/spark,apache-spark-on-k8s/spark,jlopezmalla/spark,apache-spark-on-k8s/spark,publicRoman/spark,publicRoman/spark,jlopezmalla/spark,publicRoman/spark,jlopezmalla/spark,jlopezmalla/spark,jlopezmalla/spark,publicRoman/spark,apache-spark-on-k8s/spark