rework document header

I've made some recommendations about how to layout the header.

* moved the title to the first line (it can still reference attributes defined in the header)
* set the doctype, which is semantically part of the document
* set the revnumber attribute so that it appears in the header (revnumber is the official name of the revision number in AsciiDoc)

In general, you should avoid any blank lines before the first line of content (much like HTTP).

I realize that this moves the logo under the document title. I believe that it's best to handle the order of layout using a stylesheet or a custom converter template instead of reordering the content in the source document. In fact, for a title logo, it's best to use a document attribute and then allow the converter to place it. Asciidoctor PDF has support for this. Simply set `title-logo` to the path of the logo and it will appear on the front page. There's no such support in HTML, but we could certainly add it.

Note that any content before the title (such as the logo) prevents the document from having a header or proper title.