doc/rpms: add reference to rpms doc page

The RPM page was not referenced anywhere,
only accesible via list of files or direct link.

Signed-off-by: Tomasz Zawadzki <tomasz.zawadzki@intel.com>
Change-Id: I3a72add1cc2530a73f383ca0c144a551d94083e8
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/8015


Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Shuhei Matsumoto <shuhei.matsumoto.xt@hitachi.com>
Reviewed-by: Ziye Yang <ziye.yang@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: Karol Latecki <karol.latecki@intel.com>