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
-   addrmap
-   askenv
-   base
-   bootefi
-   booti
-   bootmenu
-   button
-   x86/cbsysinfo
-   conitrace
-   echo
-   exception
-   extension
-   exit
-   false
-   fatinfo
-   fatload
-   for
-   load
-   loady
-   mbr
-   md
-   mmc
-   pinmux
-   pstore
-   qfw
-   reset
-   sbi
-   sf
-   scp03
-   setexpr
-   size
-   true
-   ums
-   wdt
+   cmd/acpi
+   cmd/addrmap
+   cmd/askenv
+   cmd/base
+   cmd/bootefi
+   cmd/booti
+   cmd/bootmenu
+   cmd/button
+   cmd/cbsysinfo
+   cmd/conitrace
+   cmd/echo
+   cmd/exception
+   cmd/extension
+   cmd/exit
+   cmd/false
+   cmd/fatinfo
+   cmd/fatload
+   cmd/for
+   cmd/load
+   cmd/loady
+   cmd/mbr
+   cmd/md
+   cmd/mmc
+   cmd/pinmux
+   cmd/pstore
+   cmd/qfw
+   cmd/reset
+   cmd/sbi
+   cmd/sf
+   cmd/scp03
+   cmd/setexpr
+   cmd/size
+   cmd/true
+   cmd/ums
+   cmd/wdt