LUD-04 seems incomplete

[AndySchroder]

Taking a look at https://github.com/lnurl/luds/blob/luds/04.md , there is no detailed explanation for the linkingKey derivation. I see there is an older document https://github.com/lnurl/luds/blob/legacy/lnurl-auth.md which contains a lot more detail.

I look harder and I find LUD-05 and LUD-13, but those documents are never mentioned in LUD-04. I think there should be some clearer ties between them. Also, seems like the graphic from the legacy document is gone. Are there any other missing LUD references that I should be looking at to more clearly understand LNURL-auth?