Use runnable code to generate main documentation samples

[philwebb]

We should look at using running sample code imported into our .adoc files to prevent issues like #6312. Spring Security has done this already.

[snicoll]

That and actually testing them would be nice to ensure backward compatibility.

[wilkinsona]

I've done it in REST Docs too. It's not too arduous.

[wilkinsona]

I've made a (tiny) start on this. See in ccaa19d.