Documentation: Add documentation for packet sockets

Signed-off-by: Zhe Weng <wengzhe@xiaomi.com>

Documentation/components/net/index.rst

@@ -7,6 +7,7 @@ Network Support
 
   sixlowpan.rst
   socketcan.rst
+  pkt.rst
   nat.rst
   netdev.rst
   netguardsize.rst

Documentation/components/net/pkt.rst

@@ -0,0 +1,37 @@
+===========================
+"Raw" packet socket support
+===========================
+
+Packet sockets (:c:macro:`AF_PACKET`) allow receiving and transmitting frames
+without a transport protocol in between. Frames received are copied into a
+packet socket tap before they enter the network. Data written into a packet
+socket will bypass the network altogether and be placed in the transmission
+buffer of the network interface driver.
+
+Configuration Options
+=====================
+
+``CONFIG_NET_PKT_PREALLOC_CONNS``
+  Number of preallocated packet connections (all tasks).
+``CONFIG_NET_PKT_ALLOC_CONNS``
+  Dynamic memory allocations for packet connections.
+``CONFIG_NET_PKT_MAX_CONNS``
+  Maximum number of packet connections.
+
+Usage
+=====
+
+.. code-block:: c
+
+  struct sockaddr_ll addr;
+  uint8_t buffer[BUFSIZE];
+  int sd = socket(AF_PACKET, SOCK_RAW, 0); /* Create a packet socket */
+
+  addr.sll_family = AF_PACKET;
+  addr.sll_ifindex = if_nametoindex("eth0");
+  bind(sd, (FAR struct sockaddr *)&addr, sizeof(addr)); /* Bind to device */
+
+  recv(sd, buffer, sizeof(buffer), 0); /* read(sd, buffer, sizeof(buffer)); */
+  send(sd, buffer, sizeof(buffer), 0); /* write(sd, buffer, sizeof(buffer)); */
+
+  close(sd); /* Close the socket */