arp: Generalize to common and add cache clearing.

pages/common/arp.md

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+# arp
+
+> Show and manipulate your system's ARP cache.
+
+- Show current arp table:
+
+`arp -a`
+
+- Clear the entire cache:
+
+`sudo arp -a -d`
+
+- Delete a specific entry:
+
+`arp -d {{address}}`
+
+- Create an entry:
+
+`arp -s {{address}} {{mac address}}`