Document accepted normal maps

## How can Bevy's documentation be improved?

I'm talking about [StandardMaterial.normal_map_texture](https://docs.rs/bevy/latest/bevy/pbr/struct.StandardMaterial.html#structfield.normal_map_texture). Strictly speaking, this is dealt with by the `image` crate and not Bevy itself, however many (most?) normal maps are exported per default as 16 bit per channel PNGs, which when used in Bevy result in a very confusing error message. I myself, as well as a few people on Discord have run into this and lost some time debugging a situation which should be a standard use-case.

Therefore I think we should add a cautionary note in the docs.

I'd open a PR for this, but as you can see in [this Discord thread](https://discord.com/channels/691052431525675048/1097564218720518144/1097564218720518144), I don't know what Bevy actually accepts as a normal map other than 8 bit per channel PNGs.