Replace TOC with manual listing

Essentially optimize for viewing on Github since the Asciidoctor
TOC works but doesn't look good with no styling.

CONTRIBUTING.adoc

 [[contributing]]
 # Contributing  to the Spring Framework
-:toc: preamble
-:toclevels: 4
 
 First off, thank you for taking the time to contribute!
 
+**Table of Contents**
+
+* <<conduct>>
+* <<how>>
+** <<how-question>>
+** <<how-ticket>>
+** <<how-pull-request>>
+* <<code-style>>
+* <<documentation>>
+
 
 
 [[conduct]]