doc: fix internal links for older releases

doc: fix internal links for older releases

Using external explicit references to http://doc.dpdk.org makes older
releases documentation point to the current master documentation pages.
Switch to internal references.

Fixes: 59ad25fe2184 ("doc: add overview of qat guide")
Fixes: 30e7fbd62839 ("doc: add event timer adapter guide")
Fixes: b7f859c9a9a5 ("doc: add switch representation documentation")
Fixes: f714a18885a6 ("app/testbbdev: add test application for bbdev")
Cc: stable@dpdk.org
Signed-off-by: David Marchand <david.marchand@redhat.com>
Acked-by: Stephen Hemminger <stephen@networkplumber.org>