NMS-7220: Documentation update asciidoctor-maven-plugin

- Updated from version 0.1.4 to 1.5.0
- Created placeholder to keep asciidoc directories in git, which are required by asciidoctor maven module
Merge pull request #179 from indigo423/NMS-7220-docs-update-asciidoctor-maven-plugin

NMS-7220: Documentation update asciidoctor-maven-plugin