Bring back Homebrew installation instructions with note about PyTorch

[simonw]

I stopped recommending Homebrew because it uses Python 3.12 and PyTorch isn't available for that yet:

• Homebrew release can't install plugins that use PyTorch (no PyTorch for Python 3.12 yet) #315 (comment)

But we have a workaround for that now:

• Homebrew release can't install plugins that use PyTorch (no PyTorch for Python 3.12 yet) #315 (comment)

So if I document that workaround I can bring back Homebrew.

See also discussion here:

• Add homebrew install instruction #370

[simonw]

https://llm.datasette.io/en/latest/setup.html#homebrew-warning