docs: move some stuff into "Networking" section

The "Networking" section has a lonely single document listed right now,
even though the "Concepts" section has two more network related docs.
Move them over, let's end this loneliness.

docs/NETWORK_ONLINE.md

@@ -1,6 +1,6 @@
 ---
 title: Running Services After the Network Is Up
-category: Concepts
+category: Networking
 layout: default
 SPDX-License-Identifier: LGPL-2.1-or-later
 ---

docs/PREDICTABLE_INTERFACE_NAMES.md

@@ -1,6 +1,6 @@
 ---
 title: Predictable Network Interface Names
-category: Concepts
+category: Networking
 layout: default
 SPDX-License-Identifier: LGPL-2.1-or-later
 ---