Move Build Targets content to CLI guides

[mehulkar]

Corollary to ember-learn/cli-guides#159

[jenweber]

The learning team & friends discussed this and we want to move forward.

The Build Targets page in the Guides should:

• have most content removed, and replaced with a link to the CLI Guides
• It should retain at least one paragraph explaining why and when someone would need this info
• It may include links to other helpful resources

The goal is that the page that remains should have at least a little rich content and not just look like a placeholder. Otherwise, the Guides will seem like a "pruned tree" when instead it should read like a book.

The CLI Guides PR should be merged first so we have something to link to.