doc: net: Add page for net_time

So that the type is visible and "referencable" in the documentation.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>

doc/connectivity/networking/api/net_time.rst

@@ -0,0 +1,9 @@
+.. _net_time_interface:
+
+Network time representation in the network stack
+################################################
+
+API Reference
+*************
+
+.. doxygengroup:: net_time

doc/connectivity/networking/api/tsn.rst

@@ -7,4 +7,5 @@ Time Sensitive Networking
    :maxdepth: 1
 
    gptp.rst
+   net_time.rst
    ptp_time.rst

include/zephyr/net/net_time.h

@@ -12,6 +12,10 @@
  * Inspired by
  * https://github.com/torvalds/linux/blob/master/include/linux/ktime.h and
  * https://github.com/torvalds/linux/blob/master/[tools/]include/linux/time64.h
+ *
+ * @defgroup net_time Network time representation.
+ * @ingroup networking
+ * @{
  */
 
 #ifndef ZEPHYR_INCLUDE_NET_NET_TIME_H_
@@ -112,4 +116,8 @@ typedef int64_t net_time_t;
 }
 #endif
 
+/**
+ * @}
+ */
+
 #endif /* ZEPHYR_INCLUDE_NET_NET_TIME_H_ */