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.
Links are always good; I'll add those.

I like the See Below phrasing because it maybe tells the first-time reader that they only need to continue reading this section in order and they'll get the relevant information.  They don't need to click a link to what might be a completely different part of the docs, read the entry, and then come back to the table.

I don't know if it actually works that way---do you recall what you thought as you read the See Below entries versus the compactSize entries which use only links?