Tweaks after last pr.

- Use exportsOverride/layout to bring in boxsizing.htc with "the CF way"
 - Can't do this in the grand CF scheme because we need to link to this file using root relative or an absolute reference and since we use github pages a lot that will change per project.
- ~~Remove Less extend statements, which do not output in the docs, in favor of simply calling the original classes, which will output. (This produces extra CSS, but gzip should nullify that.)~~ done
- ~~Finish setting media query variables.~~ done
- Less mixins and variables need meaningful docs.
 - Added some helpful info to the vars page but we need to wait for Less support in Topdoc in order to get more helpful information in here.