docs/gps: Add documentation for gps example program.

Add documentation for `gps` example program as part of #11081.

Signed-off-by: Matteo Golin <matteo.golin@gmail.com>

Documentation/applications/examples/gps/index.rst

@@ -2,4 +2,58 @@
 ``gps`` GPS example
 ===================
 
-GPS example.
+This example can be used to interact with GPS devices in NuttX. It uses the
+`MINMEA <https://github.com/kosma/minmea>`_ library to parse standard NMEA
+messages and print out GPS data to the console.
+
+To use the program, provide the character device path for the GPS serial
+connection as the only argument. If no path is provided, the program will
+default to ``/dev/ttyS1``.
+
+If you want to be able to see the floating point output of this program, make
+sure you remember to enable ``CONFIG_LIBC_FLOATINGPOINT``.
+
+The program loops forever parsing NMEA values from the serial device.
+
+Waiting for GPS Fix
+-------------------
+
+.. code:: console
+
+   nsh> gps /dev/ttyS3
+   Fixed-point Latitude...........: 0
+   Fixed-point Longitude..........: 0
+   Fixed-point Speed..............: 0
+   Floating point degree latitude.: nan
+   Floating point degree longitude: nan
+   Floating point speed...........: nan
+   Fix quality....................: 0
+   Altitude.......................: 0
+   Tracked satellites.............: 0
+
+The output will show ``nan`` and ``0`` for values while waiting to obtain a fix,
+at which point real values will begin to appear.
+
+With GPS Fix
+------------
+
+.. code:: console
+
+   nsh> gps /dev/ttyS3
+   Altitude.......................: 73172                                         
+   Tracked satellites.............: 5                                             
+   Fixed-point Latitude...........: 4628356                                       
+   Fixed-point Longitude..........: -8058408                                      
+   Fixed-point Speed..............: 110                                           
+   Floating point degree latitude.: 46.476547                                     
+   Floating point degree longitude: -80.977995                                    
+   Floating point speed...........: 0.001833                                      
+   Fix quality....................: 1                
+
+You can now see the information is filled in with the data from the GPS.
+
+.. note::
+
+   Fixed-point readings may have a different scale than the floating-point
+   readings. For instance, the altitude above is fixed-point, but is actually
+   73.172 meters in floating point.