docs: Move the gnetworking SECTION

Move it to a separate page.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>

Helps: #3037

docs/reference/gio/networking.md

@@ -0,0 +1,29 @@
+Title: gnetworking.h
+SPDX-License-Identifier: LGPL-2.1-or-later
+SPDX-FileCopyrightText: 2010 Dan Winship
+
+# gnetworking.h
+
+The `<gio/gnetworking.h>` header can be included to get
+various low-level networking-related system headers, automatically
+taking care of certain portability issues for you.
+
+This can be used, for example, if you want to call
+[`setsockopt()`](man:setsockopt(2)) on a [class@Gio.Socket].
+
+Note that while WinSock has many of the same APIs as the
+traditional UNIX socket API, most of them behave at least slightly
+differently (particularly with respect to error handling). If you
+want your code to work under both UNIX and Windows, you will need
+to take these differences into account.
+
+Also, under GNU libc, certain non-portable functions are only visible
+in the headers if you define `_GNU_SOURCE` before including them. Note
+that this symbol must be defined before including any headers, or it
+may not take effect.
+
+There is one function provided specifically for initialising the networking
+APIs, which needs to be called only if they are being used before GLib
+initialises itself:
+
+ * [func@Gio.networking_init]