Add docfx docs to GitHub Pages

[georg-jung]

I guess this is a quite opinionated thing to do, so please feel free to close if you don't like it.

Merging this would add automatic publishing of a generated API reference to GitHub pages. To properly work it would additionally require some GitHub pages setup, e.g. adding a custom domain (having a custom domain is not required though).

I personally think having this kind of API reference available would be helpful to get a quick overview of what SharpIppNext has to offer and how it works. What you get is similar to what can be achieved using Visual Studio's Object Browser or IntelliSense.

To get a first impression how this would look like, you can take a look at a similar docs page I create for one of my projects: https://fastberttokenizer.gjung.com/

Alternatively you could locally execute from repo root of this PR's branch:

dotnet tool install -g docfx
docfx docfx/docfx.json --serve

But I'd also understand if you don't want to take the additional maintenance burden. Thanks for creating a modern printing solution for the .NET ecosystem!