Finally published journal DOI added

In the project's 'metadata.conf', we also have an option to store the
journal DOI of the project (that will later be printed in the output file
products). So now that the paper's DOI has been set by the journal, it was
time to add it in the project too.

While looking at the usage of the metadata, I noticed that the "Publication
checklist" of 'README-hacking.md' didn't talk about it. In fact, the part
about putting metadata went into a lot of detail without even mentioning
the generic 'print-general-metadata' variable (previously called
'print-copyright') that is created in 'initialize.mk'. So I removed those
extra points and just recommended using this variable for plain-text files
and putting similar info in other formats.

Some other minor changes were made:

 - The metadata now doesn't need the fixed 'https://doi.org/' prefix (to
   make it consistent with the arXiv identifier). Inside 'initialize.mk',
   there are now two variables called 'doi-prefix-url' and
   'arxiv-prefix-url' that contain the fixed prefix.

 - The 'print-copyright' name was clearly outdated for all the extra
   metadata that this variable created (including the copyright). So its
   name was changed to 'print-general-metadata'.

The generic Maneage changes will be taken into Maneage after this (they
were tested here).

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