Update link to Windows installation guide

I've just released a new version of Run Jekyll on Windows and it's now got its own domain!

Also, all the info from `windows.md` is now also represented in my guide. If you want to keep/make Windows instructions fully outsourced, feel free to link to the guide directly from the info note in `installation.md`. Obviously your call though.

Thanks for linking to the guide at all, btw, I didn't even notice until recently. :blush: <3