[REQ] Add AsciiDoc as an option for client library documentation

Created by: tballast

Is your feature request related to a problem? Please describe.

We would like to generate SDK libraries, as well as a documentation website with the use of this generator. We would also like to use Antora to generate a documentation website. Antora however generates pages based on Asciidoc files, not markdown. The SDKs that are generated typically only use markdown for their documentation. I would like to be able to choose between Markdown or Asciidoc for the SDK documentation library.

Describe the solution you'd like

When I generate a Client SDK library, I would like to have a command line option that allows be to choose the language that the documentation will be generated in, such as [--documentation-lang <documentation-lang>]

Our primary interest is AsciiDoc, however if this option is present then it also opens up the ability to use other languages as well.

Some potential usages: openapi-generator generate -i petstore.yaml -g java --documentation-lang asciidoc -o /tmp/test/ openapi-generator generate -i petstore.yaml -g java --documentation-lang html -o /tmp/test/ openapi-generator generate -i petstore.yaml -g java --documentation-lang pdf -o /tmp/test/ openapi-generator generate -i petstore.yaml -g java --documentation-lang latex -o /tmp/test/

Describe alternatives you've considered

We have experimented with simply converting the already generated markdown documentation into asciidoc, however this carries it's own set of issues as most of the generated documentation uses tables, which are not officially supported in the markdown specification, and thus don't always convert properly into asciidoc.

Additional context

No additional context that I can think of.