doc: added descriptions and images for current nodes n29-n30.

doc/mcu-examples/n29/0BUILD

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+<?xml?>
+
+<gwbuild>
+
+  <extradist>
+    README.md
+    n29_pcb_back.png
+    n29_pcb_front.png
+    n29_schematic.png
+  </extradist>
+
+</gwbuild>
+

doc/mcu-examples/n29/README.md

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+
+N29
+===
+
+
+N29 is a ATtiny84 based PCB with either a temperature/humidity sensor or
+a CO2 sensor. It also contains a motion detector and a Photo-LED/transistor
+for light measurement.
+
+
+
+Overview
+--------
+
+
+- Role: Air quality and climate sensors
+- MCU: AtTiny84
+- Connection: RJ45
+- COM: COM2W
+- Predecessor: N27, N24, N23, N19
+- Periphery:
+  - PIR sensor (AMN31112)
+  - TWI interface
+    - SI7021 temperature and humidity sensor
+      or
+    - SGP-30/40 air quality sensor
+  - Photo LED
+