Resolves https://github.com/spring-projects/spring-integration/issues/3490
- also fix double import of `jmx.adoc`
- also fix `redis.adoc`
* Remove redundant repo in `docs.gradle`
* Use `stylesheet.css` for `asciidoctorj` plugin config
* Fix `stylesheet.css` to import `spring.css` from the same dir
since they both are copied over there into the same dir
* Fix link for JMX chapter from the `metrics.adoc`
* Fix some inconsistency in the `amqp.adoc`
