This commit introduces a new annotation, @AutoConfigureRestDocs,
which can be used to enable auto-configuration of Spring REST Docs.
The auto-configuration removes the need to use Spring REST Docs' JUnit
rule and will automatically configure MockMvc. Combined with the new
auto-configuration for MockMvc it allows a test class to be free of
boilerplate configuration:

@RunWith(SpringRunner.class)
@WebMvcTest
@AutoConfigureRestDocs(outputDir = "target/generated-snippets",
        uriScheme = "https", uriHost = "api.example.com",
        uriPort = 443)
public class ExampleDocumentationTests {

    @Autowired
    private MockMvc mvc;

    @Test
    public void documentIndex() {
        // …
    }

}

For more advanced customization a RestDocsMockMvcConfigurationCustomizer
bean can be used.

If a RestDocumentationResultHandler is found in the context, it will
be passed to the ConfigurableMockMvcBuilder's alwaysDo method as part
of its customization.

Closes gh-5563