Initial swag at UDP transport docs, also converted socket transport to mdbook

This has a lot of TODO links, because there are missing pieces still of course.

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+# BSD Socket Transport (Experimental)
+
+The {{i:_socket_ transport}} supports communication between
+peers across arbitrary BSD sockets, such as those that are
+created with [`nng_socket_pair()`][nng_socket_pair].
+
+This transport only supports [listeners][listener],
+using [`nng_listener_create()`][nng_listener_create].
+
+> [!NOTE]
+> Attempts to create [dialers][dialer] using this transport will result in `NNG_ENOTSUP`.
+
+The socket file descriptor is passed to the listener using
+the {{i:`NNG_OPT_SOCKET_FD`}} option (as an integer).
+Setting this option will cause the listener to create a [pipe][pipe]
+backed by the file descriptor.
+
+The protocol between peers using this transport is compatible with the protocol used
+for the [_tcp_][tcp] transport, but this is an implementation detail and subject to change without notice.
+{{footnote: Specifically it is not compatible with the [_ipc_][ipc] transport.}}
+
+> [!NOTE]
+> This transport is _experimental_, and at present is only supported on POSIX platforms.
+> {{footnote: Windows lacks a suitable `socketpair()` equivalent function we could use.}}
+
+## Registration
+
+No special action is necessary to register this transport.
+
+## URI Format
+
+This transport uses the URL {{i:`socket://`}}, without further qualification.
+
+## Socket Address
+
+The socket address will be of family {{i:`NNG_AF_UNSPEC`}}.
+There are no further socket details available.
+
+## Transport Options
+
+The following transport option is available:
+
+- {{i:`NNG_OPT_SOCKET_FD`}}: \
+  (int) \
+  \
+  This is a write-only option, that may be set multiple times on a listener.
+  Each time this is set, the listener will create a [pipe][pipe] backed by the given file
+  descriptor passed as an argument.
+
+Additionally, the following options may be supported on pipes when the platform supports them:
+
+- [`NNG_OPT_PEER_GID`][NNG_OPT_PEER_GID]
+- [`NNG_OPT_PEER_PID`][NNG_OPT_PEER_PID]
+- [`NNG_OPT_PEER_UID`][NNG_OPT_PEER_UID]
+- [`NNG_OPT_PEER_ZONEID`][NNG_OPT_PEER_ZONEID]
+
+[ipc]: [ipc.md]
+[tcp]: [tcp.md]
+[pipe]: [TODO.md]
+[listener]: [TODO.md]
+[dialer]: [TODO.md]
+[nng_sockaddr]: [TODO.md]
+[nng_listener_create]: [TODO.md]
+[nng_socket_pair]: [TODO.md]
+[NNG_OPT_LOCADDR]: [TODO.md]
+[NNG_OPT_REMADDR]: [TODO.md]
+[NNG_OPT_URL]: [TODO.md]
+[NNG_OPT_PEER_GID]: [TODO.md]
+[NNG_OPT_PEER_PID]: [TODO.md]
+[NNG_OPT_PEER_UID]: [TODO.md]
+[NNG_OPT_PEER_ZONEID]: [TODO.md]