docs/system/ppc/ppce500: Add heading for networking chapter

The sudden change of topics is slightly confusing and makes the
networking information less visible. So separate the networking chapter
to improve comprehensibility.

Signed-off-by: Bernhard Beschow <shentey@gmail.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20221003203142.24355-4-shentey@gmail.com>
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>

docs/system/ppc/ppce500.rst

@@ -146,6 +146,9 @@ You can specify a real world SoC device that QEMU has built-in support but all
 these SoCs are e500v2 based MPC85xx series, hence you cannot test anything
 built for P4080 (e500mc), P5020 (e5500) and T2080 (e6500).
 
+Networking
+----------
+
 By default a VirtIO standard PCI networking device is connected as an ethernet
 interface at PCI address 0.1.0, but we can switch that to an e1000 NIC by: