feat: use mdBook for documentation (#10)

docs/sensor/panel.md

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+# Sensor Panels
+
+The `asterctl` tool is started in sensor panel mode if the `--config` command line option is specified.
+
+Sensor panels are dynamic screens showing various sensor values. Multiple rotating panels are supported. 
+The sensor values must be provided in simple key-value text files from external scripts or tools. The `asterctl` tool
+is only responsible for rendering the panels on the embedded screen.
+
+Example panels from the AOOSTAR-X software, rendered with `asterctl` using dummy sensor values:
+
+<img src="../img/sensor_panel-01.png" alt="Sensor panel 1">
+
+<img src="../img/sensor_panel-02.png" alt="Sensor panel 1">
+
+See [custom sensor panels](custom_panel.md) for including custom panels.
+
+## Supported Features
+
+- One or multiple panels rotating in configurable interval (configuration value `setup.switchTime`).
+- Each panel can be configured with multiple sensor fields.
+  - Text sensor value fields are supported (`sensor.mode: 1`), but there are still some text size and positioning issues.
+  - Fan (2), progress (3) and pointer (4) sensor modes are being worked on and not all configuration options are working yet.
+- Each sensor field can be customized with an individual font, size, color and text alignment.
+- Panels are redrawn at a configurable interval (configuration value `setup.refresh`).
+  - Only the updated areas of the image are sent to the display for faster updates.
+
+See [Sensor Configuration](cfg/) for more information.