Convert CodeNarc rules to Groovy.

This is intended to preserve the same rules (including custom naming configuration) as the XML files.
Tweak syntax for codenarc build integration
Work on the manual readme, to hopefully make it easier for people to contribute docs.
Fix example code in readme.
Add some text to the top of each manual page effectively asking for help with the docs.
Add selected CodeNarc rules from unnecessary and unused and bring code into compliance.
Merge branch 'master' of github.com:ratpack/ratpack
Only run one parallel test fork in Travis CI.

This is the next step in trying to stamp out our sporadic "java.lang.OutOfMemoryError: unable to create new native thread" errors.
Minor fix for anchor highlighting on javadoc pages.
Some minor improvements to parsing, and correction of the javadocs after recent changes.
Fix minor mistake.
Make each heading in the manual a link to make referencing them easier.
Pull Travis CI build detection logic to the top-level build.