docs: fix MQTT figures and add MQTT bullet to Networking wrap-up

mqtt-fanout.svg: three fan-out arrows now share a common origin at
the broker's right-edge midpoint (470, 165) instead of fanning out
from three nearby Y points. Arrowhead polygons on the diagonal
arrows aligned perpendicular to each line's direction. The "forward"
label nudged off the middle arrow line.

mqtt-publish-packet.svg: shortened the top-row labels for the first
three byte sections so they fit inside their byte-column widths --
"type | flags" -> "PUBLISH", "7 bytes left" -> "= 7", "topic is
3 bytes" -> "= 3". The labels were colliding horizontally in the
previous version.

networking/wrap-up: add a bullet for the MQTT subsection between the
Encryption bullet and the closer, framing MQTT as the worked example
that ties together every layer below it.

Author: kwagyeman

docs/openmvcam/tutorial/networking/mqtt/figures/mqtt-fanout.svg

@@ -85,6 +85,18 @@ world:
   DTLS (TLS over UDP) uses the same module the same
   way.
 
+* **A real application protocol** -- MQTT as the
+  worked example of every layer below it wired
+  together. A 1-byte type-and-flags byte, a
+  variable-length remaining-length field, a length-
+  prefixed UTF-8 topic, and the payload, all
+  travelling over TCP (and optionally inside TLS) to
+  a broker that fans the message out to every
+  subscriber on the topic. The bundled :mod:`mqtt`
+  client wraps the wire format in a
+  ``connect`` / ``publish`` / ``subscribe`` API
+  small enough to read in one sitting.
+
 That is enough to write camera applications that talk
 to other machines, publish data to remote services,
 accept connections from clients on the local network,