doc: semihosting can only be once in TOC

Sphinx warns if a page is added to the table of contents twice.
Add a reference instead.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>

doc/board/emulation/index.rst

@@ -8,7 +8,6 @@ Emulation
 
    acpi
    blkdev
-   ../../usage/semihosting
    qemu-arm
    qemu-mips
    qemu-ppce500
@@ -16,3 +15,7 @@ Emulation
    qemu-sbsa
    qemu-x86
    qemu-xtensa
+
+Also see
+
+* :doc:`../../usage/semihosting`