Rewrite and update the MongoDB service article

Rewrite and update the MongoDB service article

MongoDB article had some fundamental issues.

 - Outdated Dockerfile
 - Insufficient / unclear instructions
 - Unnecessary comments
 - Failed to explain the role of Docker.io
 - Did not have a complete Dockerfile sample
 - Lacked a "learn more" section / link to Trusted Builds

This update aims to address all these issues with a complete re-write.

It also:

 - Corrects the label under which this article is/was listed on the menu

Closes https://github.com/dotcloud/docker/issues/5470

Docker-DCO-1.1-Signed-off-by: O.S. Tezer <ostezer@gmail.com> (github: ostezer)