Converted protocols to mdbook.

Started an introductory section too.

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+# PUB Protocol
+
+The {{i:*PUB* protocol}}{{hi:*PUB*}} is one half of a {{i:publisher}}/subscriber pattern.
+In this pattern, a publisher sends data, which is broadcast to all
+subscribers.
+The subscribing applications only see the data to which
+they have subscribed.
+
+The _PUB_ protocol is the publisher side, and the
+[_SUB_](sub.md) protocol is the subscriber side.
+
+> [!NOTE]
+> In this implementation, the publisher delivers all messages to all
+> subscribers.
+> The subscribers maintain their own subscriptions, and filter them locally.
+> Thus, this pattern should not be used in an attempt to reduce bandwidth
+> consumption.
+
+The topics that subscribers subscribe to is just the first part of
+the message body.
+Applications should construct their messages accordingly.
+
+## Socket Operations
+
+The [`nng_pub0_open()`][nng_pub_open] functions create a publisher socket.
+This socket may be used to send messages, but is unable to receive them.
+Attempts to receive messages will result in `NNG_ENOTSUP`.
+
+## Protocol Versions
+
+Only version 0 of this protocol is supported.
+(At the time of writing, no other versions of this protocol have been defined.)
+
+## Protocol Options
+
+The _PUB_ protocol has no protocol-specific options.
+
+## Protocol Headers
+
+The _PUB_ protocol has no protocol-specific headers.
+
+[nng_pub_open]: TODO.md