Implement docfx

[jscarle]

@drieseng @WojciechNagorski @Rob-Hague

I'd like to implement docfx as a way to automatically build documentation based on the XML docs found in the code. I think this could work really well for this project. The nice thing is that it can automatically be built in a pipeline and then published as a static website using GitHub Pages.

I recently implemented it in my own LightResults repository. After a bit of fighting, I was able to configure it so that it spits out documentation per targeted framework.

As an example, in my repository, the IActionableResult interface is only available starting with .NET 7.0. If you look at the generated documentation, it appears in the .NET 7.0 section and the .NET 8.0 section, but not in the .NET 6.0 section or the .NET Standard 2.0 section.

Once the right configuration is made, it's as easy as including a GitHub Action to republish the site whenever there are new changes (or in the case of SSH.NET, new releases).

Before I started work on this, I wanted to make sure that this was something you'd all be on-board with.

[Rob-Hague]

Sounds good to me, although I don't know much about building docs. I know a lot of people depend on the CHM file in the releases, but presumably an online version like this would suit them as well.

[bajpaikunal]

I was not able to located the CHM file in the release.