doc: net: Add information about websocket server API
doc: net: Add information about websocket server API

Websocket server side support was not mentioned in network
overview.rst document.

Signed-off-by: Jukka Rissanen <f9f077d1da4c7aa947f9864fa5a28c3588f6a45d@linux.intel.com>

finikorg/zephyr,ldts/zephyr,Vudentz/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,zephyrproject-rtos/zephyr,punitvara/zephyr,finikorg/zephyr,galak/zephyr,GiulianoFranchetto/zephyr,kraj/zephyr,nashif/zephyr,nashif/zephyr,explora26/zephyr,kraj/zephyr,nashif/zephyr,finikorg/zephyr,explora26/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,ldts/zephyr,galak/zephyr,punitvara/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,Vudentz/zephyr,galak/zephyr,kraj/zephyr,GiulianoFranchetto/zephyr,GiulianoFranchetto/zephyr,explora26/zephyr,kraj/zephyr,explora26/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,galak/zephyr,kraj/zephyr,ldts/zephyr,Vudentz/zephyr,punitvara/zephyr,punitvara/zephyr,explora26/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,punitvara/zephyr,nashif/zephyr,galak/zephyr