[infra] Document official OS/env support

[chalin]

This was discussed during the first Jan 2024 PSC meeting, in response to #1775. I'll write something up soon. Essentially its:

• Docsy use under any OS that supports Hugo
• Docsy build:
  • Linux and macOS are actively supported
  • Windows isn't actively supported, and is on a best effort basis

I'm not sure where to add this. Maybe to the User Guide's "front page" or just the README?

/cc @emckean

[emckean]

Maybe it belongs here? https://www.docsy.dev/docs/get-started/other-options/
I don't think we state anywhere the difference between "use" and "build" which is probably important :)