docs: fix mlx feature in "multiple machines" example

The example as it was would not solve, it ends with a fairly
obscure error:

```
  × failed to solve the conda requirements of 'mlx' 'osx-arm64'
  ╰─▶ Cannot solve the request because of: mlx >=0.5.0,<0.6.0 cannot be installed because there are no viable options:
      └─ mlx 0.5.0 | 0.5.0 | 0.5.0 | 0.5.0 | 0.5.0 | 0.5.1 | 0.5.1 | 0.5.1 | 0.5.1 | 0.5.1 would require
         └─ __osx >=13.3, for which no candidates were found.
```

This was already found previously in
prefix-dev#562 (comment),
and the fix is to add a minimum macOS version.
Note that the conda-forge package metadata (saying >=13.3) is also incorrect,
the 13.5 minimum added to the example comes from the MLX docs:
https://ml-explore.github.io/mlx/build/html/install.html#

While we're at it, also update to a recent MLX version.

docs/features/multi_environment.md

@@ -443,13 +443,15 @@ Dev
 
     [feature.mlx]
     platforms = ["osx-arm64"]
+    # MLX is only available on macOS >=13.5 (>14.0 is recommended)
+    system-requirements = {macos = "13.5"}
 
     [feature.mlx.tasks]
     train-model = "python train.py --mlx"
     evaluate-model = "python test.py --mlx"
 
     [feature.mlx.dependencies]
-    mlx = ">=0.5.0,<0.6.0"
+    mlx = ">=0.15.0,<0.16.0"
 
     [feature.cpu]
     platforms = ["win-64", "linux-64", "osx-64", "osx-arm64"]