Dev Docs: Add P2P Messages That Request Or Reply With Data

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.

en/developer-examples.md

@@ -30,6 +30,8 @@ title: "Developer Examples - Bitcoin"
 
 {% include example_payment_processing.md %}
 
+{% include example_p2p_networking.md %}
+
 {% include references.md %}
 
 </div>