docs: update 3n-skx job_specs for report_iterative

docs: update 3n-skx job_specs for report_iterative

Changes in vpp-3n-skx.md:
- all nfv_density moved to separate file vpp-3n-skx-nfv-density.md
- removed intel-x710: dot1q (except "./vm_vhost")
- removed drv_vfio_pci: dot1q, ipsec aes128cbc
- removed nat44, iacl, oacl

- Rebased on https://gerrit.fd.io/r/c/csit/+/27495
- *.md files moved to docs/job_specs/report_iterative/3n-skx/ and
  renamed

Change-Id: I952aeee65036919e35a2d98aaa5bb83743992966
Signed-off-by: Maciek Konstantynowicz <mkonstan@cisco.com>
(cherry picked from commit 6b5f02d7f004003e4d87d62f6add260ea126c0da)