Adds to the devel reference page detailed documentation on the following
messages: block, getblocks, getdata, getheaders, headers, inv, mempool,
merkleblock, notfound, and tx.
Adds to the devel examples page an example of requesting and parsing a
merkleblock message.
Adds to the devel docs overview pages links to the above two new
P2P sections.
Tweaks the autocrossref plugin ignore pattern to not crossref in the
middle of a GIF image name; this allows the inclusion of animated GIFs.
