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>
2 files changed