doc: move keepalive to multicore API section

The keepalive header was listed in misc section,
despite its doxygen comment is "LCore Keepalive Monitor".

Fixes: 75583b0d1efd ("eal: add keep alive monitoring")

Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
Acked-by: John McNamara <john.mcnamara@intel.com>

doc/api/doxy-api-index.md

@@ -86,6 +86,7 @@ The public API headers are grouped by topics:
   [lcore]              (@ref rte_lcore.h),
   [per-lcore]          (@ref rte_per_lcore.h),
   [service cores]      (@ref rte_service.h),
+  [keepalive]          (@ref rte_keepalive.h),
   [power/freq]         (@ref rte_power.h)
 
 - **layers**:
@@ -160,7 +161,6 @@ The public API headers are grouped by topics:
   [EAL config]         (@ref rte_eal.h),
   [common]             (@ref rte_common.h),
   [ABI compat]         (@ref rte_compat.h),
-  [keepalive]          (@ref rte_keepalive.h),
   [device metrics]     (@ref rte_metrics.h),
   [bitrate statistics] (@ref rte_bitrate.h),
   [latency statistics] (@ref rte_latencystats.h),