doc: describe QEMU emulation of block devices

* Add a new page about the emulation of block devices
* Add semihosting to the emulation index page
* Set toc maxdepth to 1 to improve readability

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/board/emulation/index.rst b/doc/board/emulation/index.rst
index b5b6c3f..932c65a 100644
--- a/doc/board/emulation/index.rst
+++ b/doc/board/emulation/index.rst
@@ -4,8 +4,10 @@
 =========
 
 .. toctree::
-   :maxdepth: 2
+   :maxdepth: 1
 
+   blkdev
+   ../../usage/semihosting
    qemu-arm
    qemu-mips
    qemu-ppce500