doc: Add guide how to use QEMU and GDB to debug NuttX

This guide is based on steps from this issue:
https://github.com/apache/nuttx/issues/8616

Documentation/guides/index.rst

@@ -8,6 +8,7 @@ Guides
   nfs.rst
   usbtrace.rst
   simulator.rst
+  qemugdb.rst
   rndis.rst
   drivers.rst
   tasktrace.rst

Documentation/guides/qemugdb.rst

@@ -0,0 +1,94 @@
+.. include:: /substitutions.rst
+.. _qemugdb:
+
+How to debug NuttX using QEMU and GDB
+=====================================
+
+This guide explains the steps needed to use QEMU and GDB to debug
+an ARM board (lm3s6965-ek), but it could be modified to work with other
+board or architecture supported by QEMU.
+
+Start configuring and compiling the lm3s6965-ek board with qemu-flat profile.
+
+Compiling
+---------
+
+#. Configure the lm3s6965-ek
+
+   There is a sample configuration to use lm3s6965-ek on QEMU.
+
+   Just use ``lm3s6965-ek:qemu-flat`` board profile for this purpose. 
+
+    .. code-block:: console
+
+       $ cd nuttx
+       $ ./tools/configure.sh lm3s6965-ek:qemu-flat
+
+#. Compile
+
+    .. code-block:: console
+
+       $ make -j
+
+Start QEMU
+----------
+
+#. You need to start QEMU using the nuttx ELF file just create above:
+
+    .. code-block:: console
+
+       $ qemu-system-arm -M lm3s6965evb -device loader,file=nuttx -serial mon:stdio -nographic -s
+       Timer with period zero, disabling
+       ABCDF
+       telnetd [4:100]
+       
+       NuttShell (NSH) NuttX-12.0.0
+       nsh>
+
+Start GDB to connect to QEMU
+----------------------------
+
+   These steps show how to connect GDB to QEMU running NuttX:
+
+    .. code-block:: console
+
+       $ gdb-multiarch -ix tools/nuttx-gdbinit nuttx
+       (gdb) target extended-remote localhost:1234
+       Remote debugging using localhost:1234
+       0x000012ee in up_mdelay (milliseconds=milliseconds@entry=250)
+           at common/arm_mdelay.c:51
+       51	      for (j = 0; j < CONFIG_BOARD_LOOPSPERMSEC; j++)
+       (gdb)
+
+#. From (gdb) prompt you can run commands to inpect NuttX:
+
+    .. code-block:: console
+
+       (gdb) info_nxthreads
+       target examined
+       _target_arch.name=armv7
+       $_target_has_fpu : 1
+       $_target_has_smp : 0
+       saved current_tcb (pid=0)
+       * 
+       0 Thread 0x20001538  (Name: Idle Task, State: Running, Priority: 0, Stack: 464/1000) PC: 0x12fc in up_mdelay()
+       saved current_tcb (pid=0)
+       
+       1 Thread 0x20005060  (Name: hpwork, State: Waiting,Semaphore, Priority: 224, Stack: 320/1992) PC: 0x47dd in work_thread()
+       saved current_tcb (pid=0)
+       
+       2 Thread 0x20005c30  (Name: nsh_main, State: Waiting,Semaphore, Priority: 100, Stack: 1016/2000) PC: 0x1 in _vectors()
+       saved current_tcb (pid=0)
+       
+       3 Thread 0x20006b40  (Name: NTP daemon, State: Waiting,Signal, Priority: 100, Stack: 864/1952) PC: 0x0 in _vectors()
+       saved current_tcb (pid=0)
+       
+       4 Thread 0x20008540  (Name: telnetd, State: Waiting,Semaphore, Priority: 100, Stack: 616/2008) PC: 0x20008fd4 in No()
+       saved current_tcb (pid=0)
+       saved current_tcb (pid=0)
+       saved current_tcb (pid=0)
+       saved current_tcb (pid=0)
+       (gdb) 
+
+As you can see QEMU and GDB are powerful tools to debug NuttX without using external board or expensive debugging hardware.
+