kfence, Documentation: add KFENCE documentation

kfence, Documentation: add KFENCE documentation

Add KFENCE documentation in dev-tools/kfence.rst, and add to index.

Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
Co-developed-by: Alexander Potapenko <glider@google.com>
Signed-off-by: Alexander Potapenko <glider@google.com>
Signed-off-by: Marco Elver <elver@google.com>
Reviewed-by: Jann Horn <jannh@google.com>
[port kfence feature to rpi-5.10.95]
Signed-off-by: Sung-hun Kim <sfoon.kim@samsung.com>
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Seung-Woo Kim <sw0312.kim@samsung.com>
Change-Id: I8747dbbd292a396b812f4255985faa95c11d74e7