Top-level main.css for all asciidoctor styles

Properly separate tocbot from asciidoctor styles and use main.css to
include both.

gradle/docs.gradle

@@ -85,7 +85,7 @@ asciidoctor {
 	}
 	resources {
 		from(sourceDir) {
-			include 'images/**', 'tocbot-3.0.2/**'
+			include 'images/*', 'stylesheets/*', 'tocbot-3.0.2/*'
 		}
 	}
 	logDocuments = true
@@ -104,7 +104,7 @@ asciidoctor {
 			sectnums: '',
 			'source-highlighter': 'coderay@', // TODO switch to 'rouge' once supported by the html5 backend
 			stylesdir: 'stylesheets/',
-			stylesheet: 'spring.css',
+			stylesheet: 'main.css',
 			'spring-version': project.version
 
 }