## What changes were proposed in this pull request?
The user guide for LSH is added to ml-features.md, with several scala/java examples in spark-examples.

## How was this patch tested?
Doc has been generated through Jekyll, and checked through manual inspection.

Author: Yunni <Euler57721@gmail.com>
Author: Yun Ni <yunn@uber.com>
Author: Joseph K. Bradley <joseph@databricks.com>
Author: Yun Ni <Euler57721@gmail.com>

Closes #15795 from Yunni/SPARK-18081-lsh-guide.

(cherry picked from commit 34777184)
Signed-off-by: Joseph K. Bradley <joseph@databricks.com>