netdb/lib_dnsdelserver.c: support delete the DNS server address

by index or address

 - dns_del_nameserver()
 - dns_del_nameserver_by_index()

Update the "DNS function" section in 11_network.rst, and create
the netlib API documentation in netlib/index.rst

Signed-off-by: nuttxs <zhaoqing.zhang@sony.com>

Author: nuttxs