Support asciidoc OOTB and enable to disable markdown

[rmannibucau]

Hi,

Markdig is a great Markdown implementation but as with any markdown implementation you end up writing a Markdig documentation and not a Markdown doc.
This has several pitfalls in documentation pipelines like not being integrable with some 3rd party tools - without rewriting.

Ideally docfx could use a portable documentation language.

My proposal would be Asciidoc which is very complete (tables, definition lists, ...), extensible by design - which means you don't need the same level of abstraction than Markdig with nested parser for any extension, just use a block which is parsed by default for ex with a specific type.

I ported a simple Java implementation I did in c# at https://github.com/rmannibucau/NAsciidoc/ to let you test it withing docfx and judge - I assume you would either reimplement it or fork it to integrate it but happy to just move there if you want to use it as a dep.

The idea is to check what it means for docfx and provide to doc writers a more portable language (js=asciidoctor.js, ruby=asciidoctor, java=yupiik-asciidoc or asciidoctor, python=asciidoc probably).

For me the selection would be done based on the source extension (.asciidoc or .adoc -> asciidoc, .md or .markdown -> markdig).

Hope it makes sense.