Improve grids documentation

This commit improves Grids documentation by doing the following:

* **Simplifying the examples**: Examples are more concise now to make it easier for newbies to understand.
* **Simplifying grid rules**: Grid rules have been coalesced into one section with 3 simple points so that it's easier to understand.
* **Distinguishing inline examples from actual text**: Examples have an inverse color scheme so they stand out amongst the actual text on the page.
* **Add section on using custom fonts**: I added a section on how to integrate custom fonts with a grid, in case people run into http://github.com/yui/pure/issues/41/