Add documentation about toktx dependency about gltf-transform

[donmccurdy]

Thanks for mentioning this — is there somewhere in the docs that you would intuitively look for this information? Maybe it should just show a warning if you try to convert textures without the dependency installed?

And just to confirm — was it just conversion to KTX that failed when you didn't have the dependency, or a larger error?

[donmccurdy]

Should be fixed on next release, thanks!

[vorg]

The command exist check is good. Adding small note in https://gltf-transform.donmccurdy.com/cli.html installation section would be another place.