Asciidoctor: Create HTML/PDF with TOC level 5

The default is 2 in Asciidoctor. A higher TOC level enables developers
and users to post more precise deep-links into docs. In PDF versions, it
is also easier now to navigate from the TOC to a specific subchapter.

Signed-off-by: Alexander Kriegisch <Alexander@Kriegisch.name>

pom.xml

@@ -513,6 +513,7 @@
 						<attributes>
 							<source-highlighter>coderay</source-highlighter>
 							<toc>left</toc>
+							<toclevels>5</toclevels>
 							<icons>font</icons>
 							<!--
 								Create separate CSS files, rather than embedding CSS into each HTML file