Rather than introducing a custom macro, this commit opts to implicitly
configure the snippets attribute instead. The attribute is configured
will the path into which snippets are generated, relative to the
directory that contains the Asciidoctor document that is being
rendered.
The samples and documentation have been updated to use the new
spring-restdocs-asciidoctor module and the implicitly configured
snippets attribute.
Closes gh-297
