[C#] Show API docs on arrow.apache.org

[asfimport]

We should use the XML documentation files that are generated during the build and publish them to arrow.apache.org under the Documentation tab.

One option would be to use https://ejball.com/XmlDocMarkdown/ to generate a markdown file, which could be displayed as a web page.

Another would be to use https://dotnet.github.io/docfx/.

Reporter: Eric Erhardt / @eerhardt

_{Note: This issue was originally created as ARROW-9406. Please see the migration documentation for further details.}

[asfimport]

Kouhei Sutou / @kou:
We're generating documents by https://github.com/apache/arrow/blob/master/ci/scripts/docs_build.sh .
If we can generate HTML documentation for C#, we will be able to use it in docs_build.sh easily.

[asfimport]

Joris Van den Bossche / @jorisvandenbossche:
There might be some projects to plug the XML into sphinx, but the ones I found from just googling (I am not familiar with C# otherwise) don't seem very active: https://github.com/readthedocs/sphinxcontrib-dotnetdomain, https://hach-que.github.io/SphinxForDotNet/ (https://github.com/hach-que/SphinxForDotNet)

[asfimport]

Neal Richardson / @nealrichardson:
It's trivial (if the docs exist) to add links to them to the existing sphinx docs and website. We do this for other languages already.