Write an overview chapter

[mark-i-m]

cc @lokalmatador

Copying from the dropbox doc:

More or less an expansion of the "High-level overview of the compiler source" section)
Content (or links to):

• compiler high level structure
• system requirements & dependencies
• compiler module structure
• crate dependencies
• compiler configuration
• compiler inputs
• compiler outputs
• compiler state
• Walk-through of compiling a meaningful example

Current plans are to walk through a simple example program and show how it moves through the compiler.

This issue has been assigned to @lokalmatador via this comment.

[spastorino]

@rustbot assign @lokalmatador