[SPARK-14742][DOCS] Redirect spark-ec2 doc to new location

## What changes were proposed in this pull request?

Restore `ec2-scripts.md` as a redirect to amplab/spark-ec2 docs

## How was this patch tested?

`jekyll build` and checked with the browser

Author: Sean Owen <sowen@cloudera.com>

Closes #12534 from srowen/SPARK-14742.

docs/ec2-scripts.md

+---		
+layout: global
+title: Running Spark on EC2
+redirect: https://github.com/amplab/spark-ec2#readme
+---
+
+This document has been superseded and replaced by documentation at https://github.com/amplab/spark-ec2#readme