doc: usage: Group all shell command docs into cmd/ sub-directory

Currently all shell command docs are put in the doc/usage root.
Let's group them into cmd/ sub-directory.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>

doc/usage/cmd/md.rst

@@ -102,5 +102,3 @@ Return value
 ------------
 
 The return value $? is always 0 (true).
-
-

doc/usage/index.rst

@@ -18,38 +18,38 @@ Shell commands
 .. toctree::
    :maxdepth: 1
 
-   acpi
+   cmd/acpi
-   addrmap
+   cmd/addrmap
-   askenv
+   cmd/askenv
-   base
+   cmd/base
-   bootefi
+   cmd/bootefi
-   booti
+   cmd/booti
-   bootmenu
+   cmd/bootmenu
-   button
+   cmd/button
-   x86/cbsysinfo
+   cmd/cbsysinfo
-   conitrace
+   cmd/conitrace
-   echo
+   cmd/echo
-   exception
+   cmd/exception
-   extension
+   cmd/extension
-   exit
+   cmd/exit
-   false
+   cmd/false
-   fatinfo
+   cmd/fatinfo
-   fatload
+   cmd/fatload
-   for
+   cmd/for
-   load
+   cmd/load
-   loady
+   cmd/loady
-   mbr
+   cmd/mbr
-   md
+   cmd/md
-   mmc
+   cmd/mmc
-   pinmux
+   cmd/pinmux
-   pstore
+   cmd/pstore
-   qfw
+   cmd/qfw
-   reset
+   cmd/reset
-   sbi
+   cmd/sbi
-   sf
+   cmd/sf
-   scp03
+   cmd/scp03
-   setexpr
+   cmd/setexpr
-   size
+   cmd/size
-   true
+   cmd/true
-   ums
+   cmd/ums
-   wdt
+   cmd/wdt