Docs build simplification

[abelsromero]

Checking #802 I've identified a few things that can be removed:

• prepareAsciidocBuild is not needed, all docs reside in the same folder and UI them is injected from an Asciidoctor extension. This will require updating examples-dir attribute in docs.
• examples-dir should be passed from Gradle build script. The value is duplicated in several sources and furthermore attributes are not valid above the main section title. Asciidoctor allows that though but should not be done.
• eruby option is only required when using templateDirs which we do not use.
• Several attributes just apply the default value (oe.)
• UI-related attributes like stylesdir, stylesheet or source-highlighter may be added by the extension, need to test though.
• sources option in plugin uses default value.