Add Asciidoctor coding standard

[kychua]

Guidelines for compatibility with GitHub preview:

• use :imagesDir: in document header instead of build.gradle so GitHub knows where to find the images
• handling links to other .adoc files: use <<UserGuide#,User Guide>> instead of <<UserGuide.adoc, User Guide>> or <<UserGuide.html, User Guide>>, with ifdef::env-github,env-browser[:outfilesuffix: .adoc] in document header
• handling relative links where directory structure is different on GitHub pages & GitHub preview: use ifdef::env-github,env-browser[:relfileprefix: docs/] in document header
• handling section links: set idprefix: ''