doc: net: Add DHCPv6 information

DHCPv6 description was missing from the documentation.

Fixes #73290

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>

doc/connectivity/networking/api/dhcpv6.rst

@@ -0,0 +1,26 @@
+.. _dhcpv6_interface:
+
+DHCPv6
+######
+
+.. contents::
+    :local:
+    :depth: 2
+
+Overview
+********
+
+The Dynamic Host Configuration Protocol (DHCP) for IPv6 is a network management protocol
+used on IPv6 based networks. A DHCPv6 server dynamically assigns an IPv6 address
+and other network configuration parameters to each device on a network so they
+can communicate with other IP networks.
+See this
+`DHCPv6 Wikipedia article <https://en.wikipedia.org/wiki/DHCPv6>`_
+for a detailed overview of how DHCPv6 works.
+
+Note that Zephyr only supports DHCPv6 client functionality.
+
+API Reference
+*************
+
+.. doxygengroup:: dhcpv6

doc/connectivity/networking/api/system_mgmt.rst

@@ -8,6 +8,7 @@ Network System Management
 
    net_config.rst
    dhcpv4.rst
+   dhcpv6.rst
    net_hostname.rst
    net_core.rst
    net_if.rst