Make the extension Antora aware

[ggrossetie]

The extension should automatically detect an Antora documentation component and resolve external resources (images, partials, examples...).

[rgleason]

Antora uses nav.adoc files for the main index or outline. These often reside one directory above the other adoc files. Example: using GitHub/development/modules/ROOT/pages/
Also see https://github.com/opencpn-manuals/development for more details

nav.adoc - file
attachments - folder
images - folder
pages - folder

about.adoc (under pages) - asciidocbrowser nav.adoc links fail due to missing "/pages/"
info.adoc (under pages) - asciidocbrowser nav.adoc links fail due to missing "/pages/"

However we do not want to mess up the Antora output to html!!!

The links in nav.doc are not referencing the "/page/"

Example: github/development/modules/ROOT/about.adoc fails in the ascii browser extension

however these links work when Antora has run to output html and you use the html output.