man: include numeric prefixes in example file names for .link, .netdev, .network files

In order to avoid confusion with the default files we ship, let's use a
low prefix number for all examples.

Fixes #1409.

man/systemd.network.xml

@@ -893,7 +893,7 @@ DHCP=yes</programlisting>
     </example>
 
     <example>
-      <title>/etc/systemd/network/bridge-static.network</title>
+      <title>/etc/systemd/network/25-bridge-static.network</title>
 
       <programlisting>[Match]
 Name=bridge0
@@ -905,7 +905,7 @@ DNS=192.168.0.1</programlisting>
     </example>
 
     <example>
-      <title>/etc/systemd/network/bridge-slave-interface.network</title>
+      <title>/etc/systemd/network/25-bridge-slave-interface.network</title>
 
       <programlisting>[Match]
 Name=enp2s0
@@ -914,7 +914,7 @@ Name=enp2s0
 Bridge=bridge0</programlisting>
     </example>
     <example>
-      <title>/etc/systemd/network/ipip.network</title>
+      <title>/etc/systemd/network/25-ipip.network</title>
 
       <programlisting>[Match]
 Name=em1
@@ -924,7 +924,7 @@ Tunnel=ipip-tun</programlisting>
     </example>
 
     <example>
-      <title>/etc/systemd/network/sit.network</title>
+      <title>/etc/systemd/network/25-sit.network</title>
 
       <programlisting>[Match]
 Name=em1
@@ -934,7 +934,7 @@ Tunnel=sit-tun</programlisting>
     </example>
 
     <example>
-      <title>/etc/systemd/network/gre.network</title>
+      <title>/etc/systemd/network/25-gre.network</title>
 
       <programlisting>[Match]
 Name=em1
@@ -944,7 +944,7 @@ Tunnel=gre-tun</programlisting>
     </example>
 
     <example>
-      <title>/etc/systemd/network/vti.network</title>
+      <title>/etc/systemd/network/25-vti.network</title>
 
       <programlisting>[Match]
 Name=em1
@@ -954,7 +954,7 @@ Tunnel=vti-tun</programlisting>
     </example>
 
     <example>
-      <title>/etc/systemd/network/bond.network</title>
+      <title>/etc/systemd/network/25-bond.network</title>
 
       <programlisting>[Match]
 Name=bond1