Add Asciidoc documentation generator

[craicoverflow]

This PR adds a documentation generator for Asciidoc.

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This PR is being marked as stale due to a long period of inactivity

[marckhouzam]

I don't have an opinion on how valuable such an improvement would be compared to the extra maintenance. I'll stil re-open until we can take an official stance on it.

Thanks @craicoverflow for your suggestion and contribution!

[bartoszmajsak]

I think it would help projects using tools like Antora to embed CLI docs easily. Right now I have to use pandoc to convert from Markdown to Asciidoc which makes the process a bit cumbersome.

Thanks @craicoverflow for great work! I hope it will be merged soon :)

[zregvart]

For anyone looking for a solution to having CLI documentation in Antora, I've recently done this by implementing an Antora extension. Have a look at this pull request. We use the YAML files as input and Handlebars templates to generate Asciidoc pages and place them in Antora catalog.