Fix additional guidelines links

[OR13]

https://www.w3.org/TR/did-imp-guide/#additional-guidelines

link to https://www.w3.org/reports/did-sec-priv-criteria-overview-public.pdf is 404.

Yet it is in the repo here: https://github.com/w3c/did-imp-guide/blob/main/reports/did-sec-priv-criteria-overview-public.pdf

Ideally we can point to this documented hosted on a remote server.