doc: Add to Doxygen documentation guidelines
doc: Add to Doxygen documentation guidelines

and update the table of contents.

Co-authored-by: Jon Layton <b1c1d8736f20db3fb6c1c66bb1455ed43909f0d8@jonl.io>

peercoin/peercoin,DigitalPandacoin/pandacoin,peercoin/peercoin,DigitalPandacoin/pandacoin,peercoin/peercoin,DigitalPandacoin/pandacoin,peercoin/peercoin,peercoin/peercoin,DigitalPandacoin/pandacoin,DigitalPandacoin/pandacoin,peercoin/peercoin,DigitalPandacoin/pandacoin