doc: lacp: shorten, point to stp.md for the trap

The trap is described with STP, so the LACP page only says that it is
used. The LACP sections leave link_aggregation.md, which now points to
lacp.md, and the timing section gives the values in seconds and the
802.3ad requirements without the test history.

doc/link_aggregation.md

@@ -227,3 +227,7 @@ iperf Done.
 ```
 As you can see, the total throughput was 4.71 GBit/sec which is close to the
 maximum possible with a single 5GBit link.
+
+## LACP
+
+Dynamic aggregation with LACP is described in [lacp.md](lacp.md).