Enable documentation for `v1` and `v2`

[linkdotnet]

We need to set up our documentation so that it is easy for users to discover v1 and v2 of bUnit.

[linkdotnet]

Seems like v3 of docfx could help us out: https://github.com/dotnet/docfx/blob/v3/docs/designs/conceptual-versioning.md

But: that would also mean we would have to move the docs into their own repository to make that work (like dotnet does). Currently the cool version toggle one can find on the MS sites, is closed source and will come with v3.

The big problem: No one knows when v3 is coming. (See dotnet/docfx#2299)

[linkdotnet]

There is a way of making that work at least for the articles. Nodatime does the same: https://github.com/nodatime/nodatime.org/tree/main/build/docfx/docfx-web.json

So the general idea might be:

1. Have a new repository that is kind of a meta-repository for the docs
2. It has main and v2 (and later main and v1) as git submodules
3. We build v1 and v2 normally like today to get all the documents under _site
4. We add a new docfx.json that does the mapping of versions. (see nodatime.org stuff)
5. We use the newly generated stuff to push our docs to bunit.dev.