Document supported operating systems.

### How can we improve the documentation?

I've searched both the [documentation website](https://asdf-vm.com) and GitHub issues and I see no clear documentation as to what operating systems are supported.

I'm guessing that, based on the table [on the Getting Started page](https://asdf-vm.com/guide/getting-started.html) and relevant GitHub issues that I _have_ found, that Windows is totally not supported but WSL2 is mostly supported, but I'm still unsure of any of that because it's not explicitly stated anywhere.

Start with the documentation website, especially the Getting Started page, and review the relevant GitHub issues mentioned in the report. Establish the supported operating-system statuses, including Windows and WSL2, then document them explicitly so users can determine support without interpreting scattered references.

Written by the indexing model from the issue text.