Default Asset `key`ed transformations not documented anywhere

[kheiner]

https://github.com/directus/directus/blob/c305be58168012cfa5ae8bf3974264a61fb051d1/api/src/constants.ts#L10-L41

Makes sense to add it here:
https://directus.com/docs/guides/files/transform#preset-transformations

[linear-code]

EDU-527