Add description of hardware components.

Author: dhebbeker

doc/hardware-architecture.md

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+Hardware Architecture {#hardware_architecture}
+=====================
+
+The Task-Tracker-Device consists of hardware which is described in this chapter.
+
+The coarse structure of the hardware is sketched in the following diagram:
+
+![diagram showing the hardware structure](hardware-structure.svg)
+
+- The **microcontroller** block represents the used microcomputer including its immediate external components which are necessary for general operation.
+  This block is realized by a the development board [ESP32-S3-DevKitC-1](https://docs.espressif.com/projects/esp-idf/en/latest/esp32s3/hw-reference/esp32s3/user-guide-devkitc-1.html).
+  It includes a UART to USB transciever for convenient connection to a host system.
+  A DC voltage converter from 5V to 3.3V is also included.
+- The **power and battery charger** provides an integrated limited power source.
+  It is capable to provide uninterrupted power for the other components while charging or disconnected.
+- A **USB** connector is used for power and data.
+  Serial communication is transported over USB data.
+  The microcontroller also acts as a USB mass storage device to a USB host.
+- A OLED **display** is connected via I2C to the microcontroller.
+  It has 124x64 monochrome pixels.
+- A real-time clock (**RTC**) provides the current wall-clock time to the microcontroller.
+- **LEDs** and a **buzzer** are use to advertise states or events to the user.
+- **Buttons** can be used by the user to directly interact with the device.