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.

gradle/docs.gradle

@@ -155,6 +155,9 @@ asciidoctorj {
 asciidoctor {
 	baseDirFollowsSourceDir()
 	configurations 'asciidoctorExt'
+	attributes([
+			noheader: '' // custom header in docinfo-header.html
+	])
 	sources {
 		include '*.adoc'
 	}

src/docs/asciidoc/docinfo-header.html

+<div id="header">
+<h1>Spring Framework Documentation</h1>
+<span><strong>{revnumber}</strong></span>
+</div>