The fact that Candle supports Metal is not mentioned in the mdbook (installation)

README of this repo directs me to this page to install Candle:
https://huggingface.github.io/candle/guide/installation.html
However, the fact that Candle supports Metal is not mentioned in that page. It's only mentioned:
```
cargo add --git https://github.com/huggingface/candle.git candle-core --features "cuda"
```
So I suggest that the following should be added:
```
cargo add --git https://github.com/huggingface/candle.git candle-core --features "metal"
```

Open the installation page linked from the repository README and compare its existing CUDA example with the requested Metal installation command. Update the installation documentation to mention Candle's Metal support and show the command, then verify the rendered mdbook page contains the new example.

Written by the indexing model from the issue text.