Converted protocols to mdbook.

Started an introductory section too.

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+# PUSH protocol
+
+## DESCRIPTION
+
+The {{i:*PUSH* protocol}}{{hi:*PUSH*}} is one half of a {{i:pipeline pattern}}.
+The other side is the [_PULL_][pull] protocol.
+
+In the pipeline pattern, pushers distribute messages to pullers.
+Each message sent by a pusher will be sent to one of its peer pullers,
+chosen in a round-robin fashion
+from the set of connected peers available for receiving.
+This property makes this pattern useful in {{i:load-balancing}} scenarios.
+
+### Socket Operations
+
+The [`nng_push0_open`][nng_push_open] call creates a _PUSH_ socket.
+This socket may be used to send messages, but is unable to receive them.
+Attempts to receive messages will result in `NNG_ENOTSUP`.
+
+Send operations will observe flow control (back-pressure), so that
+only peers capable of accepting a message will be considered.
+If no peer is available to receive a message, then the send operation will
+wait until one is available, or the operation times out.
+
+> [!NOTE]
+> Although the pipeline protocol honors flow control, and attempts
+> to avoid dropping messages, no guarantee of delivery is made.
+> Furthermore, as there is no capability for message acknowledgment,
+> applications that need reliable delivery are encouraged to consider the
+> [_REQ_][req] protocol instead.
+
+### 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
+
+- [`NNG_OPT_SENDBUF`][NNG_OPT_SENDBUF]:
+  (`int`, 0 - 8192)
+  Normally this is set to zero, indicating that send operations are unbuffered.
+  In unbuffered operation, send operations will wait until a suitable peer is available to receive the message.
+  If this is set to a positive value (up to 8192), then an intermediate buffer is
+  provided for the socket with the specified depth (in messages).
+
+> [!NOTE]
+> Transport layer buffering may occur in addition to any socket
+> buffer determined by this option.
+
+### Protocol Headers
+
+The _PUSH_ protocol has no protocol-specific headers.
+
+[nng_push_open]: TODO.md
+[NNG_OPT_SENDBUF]: TODO.md
+[pull]: ./pull.md
+[req]: ./req.md