Bug#1128883: developers-reference: replace sources.list by debian.sources file format

I'm with David. 
 
Better to have a working guidance (probably with a note saying that this will change in the future) instead of having a non-working guidance with a note explaining why it breaks, if following the guidance. 
 
We do have both names (.pgp plus a .gpg-symlink) in trixie for the transition period and with the next release (forky?) the upgrade guidance in the release notes will change this to .pgp, so the additional .gpg-symlink will no longer be needed. 
 
hede