SPARK-715: Adds instructions for building with Maven to documentation

Fixes SPARK-715 (https://spark-project.atlassian.net/browse/SPARK-715)

Basically I copied a tweaked version of the instructions in this pull request #310 to a new page in the docs and added links to that page from the appropriate places.

Adds links to new instructions in:

The main Spark project README.md
The docs nav menu called "More"
The docs Overview page under the "Building" and "Where to Go from Here" sections