Improve grids documentation

This commit improves grid docs by simplifying
the examples, adding a section on fixing
github.com/yui/pure/issues/41/, and distinguishing
examples from the page.
add fix proposed in https://github.com/yui/pure/issues/41#issuecomment-22147419
reset font-stack with `serif`
Improve grid docs further

- Remove yui3 grids link
- Add section on helper classes
- Call out custom font issue at the top of the page
Minor copy edits to new Grids docs
Merge pull request #14 from ericf/tilomitra-grids-docs

Minor copy edits to new Grids docs
Merge branch 'tilomitra-grids-docs'

Conflicts:
	views/pages/grids.handlebars
Adjust use of heading levels in Grids docs