Enhance documentation to allow switching between different examples

[wilkinsona]

The documentation currently shows how to configure Spring REST Docs with either Maven or Gradle. It does so in two separate sections. There's a lot of repetition between the two sections in terms of what needs to be done, it's just how it's done that varies.

To avoid the above-described repetition, the documentation should be enhanced to allow users to switch between a code block that shows a Maven example and a code block that shows a Gradle example. This switching will also be useful for documenting support for Rest Assured (see #102).