Stop using asciidoctor compat-mode

[nik9000]

Right now we use Asciidoctor's compat-mode feature to ease our transition from AsciiDoc. It isn't complete compatibility, but it sure helps. We should transition away from using it somehow. Eventually.

[hackery]

In particular, there are places in the documentation where contributors have used "new" syntax, which renders incorrectly with compat-mode on.

e.g. Logstash Field References Deep Dive shows references like +[[deep][nesting]][field]+ but the enclosing + signs are spurious (and suggest some field reference syntax that's not described and doesn't exist).

The source in field-reference.asciidoc renders correctly when viewed on Github (which isn't using compat-mode) but the raw version shows where the "new" syntax is being used: `+ ... +`

[gtback]

Thanks, @hackery. That's a good datapoint.