net: doc: Remove Network Connectivity API page

The Network Connectivity API page is kind of pointless, as it provides
little information (which can be found elsewhere as well) and it was
pointer out that it kind of creates noise in the documentation as it can
be confused with the page which provides a summary of network APIs.

Therefore, remove the page, and move the information from it elsewhere.
The brief summary of what APIs to use was moved the Networking APIs
index page. The information about the samples demonstrating BSD sockets
API usage was moved to the BSD sockets documentation page itself.

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