Introduce custom header for HTML5 ref docs

This commit disables the automatic header generation from Asciidoc's
default template and replaces it with a custom header that omits the
"Version" label before the revision number. The revision number is also
displayed in a simple span tag in order to reduce the amount of
vertical space taken by the header.

src/docs/asciidoc/docinfo-header.html

@@ -0,0 +1,4 @@
+<div id="header">
+<h1>Spring Framework Documentation</h1>
+<span><strong>{revnumber}</strong></span>
+</div>