Created a sample header: sample_header.xml and linked it from the markup documentation page. Updated the sitemap so that plain xml docs like this can be delivered, and tweaked the markup page, adding a link to the sample header. Tag documentation should now be complete, pending any elaboration of the markup by adoption of new tags.

The simplicity of the tag structure leads me to believe we could cut the size of the schema considerably, if we put our minds to it. However, it's a bit early in the project to be removing options we might end up wanting to use.