Dev Docs: Detail Transaction Format

Preview: http://dg0.dtrt.org/en/developer-reference#raw-transaction-format

Provides a detailed description of the transaction format, replacing an
example hexdump taken from the wiki.

I'm putting this in the transaction section as the format is necessary
for the creation of txids, which are used as merkle leaves (so are
covered by consensus rules).  However, this is also the format used by
several P2P network messages to transmit transactions, so I'll be
linking back to it from there as I document those messages.