feat: use mdBook for documentation (#10)

docs/sensor/README.md

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+# Sensor Panels
+
+- [Sensor panels](panel.md)
+- [Custom sensor panels](custom_panel.md)
+- [Configuration](cfg/)
+
+## Sensor Modes
+
+Different sensor modes are supported:
+
+- [Sensor mode 1: Text](cfg/mode1_text.md)
+- [Sensor mode 2: Circular Progress](cfg/mode2_circular.md)
+- [Sensor mode 3: Progress](cfg/mode3_progress.md)
+- [Sensor mode 4: Pointer](cfg/mode4_pointer.md)
+
+## Sensor Data Sources
+
+The sensor value reading is separated from the `asterctl` tool.
+
+Sensor values are provided in separate text files and are automatically read when the file changes.  
+Only the file data source is supported at the moment, other sources like pipes, sockets etc. might be supported later.
+
+- [Text file data source](provider/text_file.md)
+
+### Sensor Data Providers
+
+- Proof of concept [Linux shell scripts](provider/shell_scripts.md)
+- [sysinfo tool](provider/sysinfo.md)