Doc: Add initial documentation to the simulator

Documentation/platforms/sim/index.rst

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+==========
+Simulators
+==========
+
+The following Simulator/Emulators are supported:
+
+.. toctree::
+   :maxdepth: 2
+   :glob:
+
+   */*

Documentation/platforms/sim/sim/index.rst

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+===
+SIM
+===
+
+It is possible to run NuttX in a simulator called ``sim``, but some features
+currently are supported only on Linux host (i.e.: Bluetooth, I2C, SPI, etc).
+
+Using ``sim`` you can test many of NuttX features without a supported board.
+Examples of supported features: Audio, Bluetooth, ELF, I2C, SPI, LVGL, Flash
+File System, NX Serves, NX Demos, NX Window Manager, ROMFS, Network: TCP,
+UDP, IP,6LoWPAN, and many more.
+
+Toolchain
+=========
+
+All you need is your machine ``gcc`` working.
+
+Compiling
+=========
+
+All you need to do is select your desired board profile configuratioon
+(see: nuttx/sim/sim/sim/configs for the listing) ::
+
+    $ ./tools/configure.sh sim:nsh
+
+    $ make
+
+
+Running
+=======
+
+When the compilation finishes it will create a ``nuttx`` binary, then run it::
+
+    $ ./nuttx 
+    login: admin
+    password: Administrator
+    User Logged-in!
+
+    NuttShell (NSH) NuttX-10.1.0
+    MOTD: username=admin password=Administrator
+    nsh> ?
+    help usage:  help [-v] [<cmd>]
+
+      .         cd        echo      hexdump   mkfatfs   pwd       source    unset     
+      [         cp        exec      kill      mkrd      readlink  test      usleep    
+      ?         cmp       exit      losetup   mount     rm        time      xd        
+      basename  dirname   false     ln        mv        rmdir     true      
+      break     dd        free      ls        poweroff  set       uname     
+      cat       df        help      mkdir     ps        sleep     umount    
+
+    Builtin Apps:
+      sh     hello  nsh    
+    nsh> uname -a
+    NuttX 10.1.0 508215581f Sep  3 2021 10:47:34 sim sim
+    nsh>
+