[docs] First cut at recommending fud2 throughout

[sampsyo]

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Here's a quick attempt to convert all the low-hanging fruit I could find in the docs to refer to fud2 instead of fud. I haven't done anything about the actual fud docs themselves, the "experimental" label, or anything like that.

Most of the changes are pretty mechanical. The only substantial bit of new writing is in the "getting started" guide, where I added a compressed overview of how to install fud2 (which is of course already explained in more detail on the main fud2 docs page).

I'm sure there's plenty more to do here, but I thought I'd just get the ball rolling!