doc: add missing referenced link

Build is failing because bsdlib/README.rst is referring to a link which
is currently not available in the links.txt file, so add it.

Signed-off-by: Fabio Utzig <fabio.utzig@nordicsemi.no>

doc/links.txt

@@ -24,6 +24,8 @@
 
 .. ### Other links
 
+.. _`Beej's Guide to Network Programming`: https://beej.us/guide/bgnet/
+
 .. _`Socket API`: https://pubs.opengroup.org/onlinepubs/9699919799/functions/socket.html
 
 .. _`ZBOSS API documentation`: https://developer.nordicsemi.com/nRF_Connect_SDK/doc/zboss_api_doc/index.html
@@ -36,4 +38,4 @@
 
 .. _`NMEA`: https://www.gpsinformation.org/dale/nmea.htm
 
-.. _`A-GPS`: https://en.wikipedia.org/wiki/Assisted_GPS
+.. _`A-GPS`: https://en.wikipedia.org/wiki/Assisted_GPS