[wp-docs] Complete idiot's installation guide

[Kaf Oseo]

This is a nice doc (looks like a lot of time was spent on it), but as
the initial focus is placed on setting up a wp.com blog, I have a wee
problem linking it from the Codex. Brandon, could you split this into
two documents, one for WordPress installation and one on wp.com?

We constantly try to define where the line for support of WordPress
(the software) ends and wp.com begins, and something like this will
only help to blur it.

-Kaf