[SPARK-17445][DOCS] Reference an ASF page as the main place to find third-party packages

## What changes were proposed in this pull request?

Point references to spark-packages.org to https://cwiki.apache.org/confluence/display/SPARK/Third+Party+Projects

This will be accompanied by a parallel change to the spark-website repo, and additional changes to this wiki.

## How was this patch tested?

Jenkins tests.

Author: Sean Owen <sowen@cloudera.com>

Closes #15075 from srowen/SPARK-17445.