revisions to AsciidoctorJ 1.5.0 announcement

Overall, it looks great!

Changes include:

* Add more context to lead paragraph
* Move “What is AsciidoctorJ?” to a sidebar
* Use the `uri-` prefix convention for the names of URI attributes
* Add content about 1.5.0 migration in general
* Add a TODO to mention the change in artifactId

Consider moving the release notes to release-notes/asciidoctorj-1-5-0.adoc and including it into the post using an include directive. See what I did in news/asciidoctor-1-5-0.adoc for reference.

We should also include the artifactId table that we did in earlier posts. See http://asciidoctor.org/news/2013/06/23/asciidoctor-maven-plugin-0-1-3-1-released