feat: Initial support for fan-, progress-, & pointer-sensors (#8)

* feat: support progress, fan, pointer sensor types

Initial implementation, not yet fully working.

* only use one rendering layer per sensor type instead one per sensor

Each layer uses > 1 MB and a panel can contain easily over 20 sensors!

* support custom panels with --panels cli argument

This includes one or more additional custom panels into the base
configuration. Multiple --panels arguments are supported

* update docs

doc/sensor_data_shell.md

@@ -0,0 +1,29 @@
+# Sensor Data Provider Shell Scripts
+
+The [/linux/scripts](../linux/scripts) directory contains some proof-of-concept Linux shell scripts.
+
+CPU and memory usage are written into a sensor data source text file that can be used by `asterctl`.
+
+```
+./cpu_usage.sh -h
+Simple PoC script to periodically write the CPU usage into a sensor text file.
+
+Usage:
+./cpu_usage.sh [-r REFRESH] [-s SENSOR_FILE] [-t TEMP_DIR]
+
+  -r REFRESH     refresh in seconds.  Default: 1
+  -s SENSOR_FILE output sensor file.  Default: /tmp/sensors/cpu.txt
+  -t TEMP_DIR    temporary directory. Default: /tmp
+```
+
+```
+./mem_usage.sh -h
+Simple PoC script to periodically write the memory usage into a sensor text file.
+
+Usage:
+./mem_usage.sh [-r REFRESH] [-s SENSOR_FILE] [-t TEMP_DIR]
+
+  -r REFRESH     refresh in seconds.  Default: 5
+  -s SENSOR_FILE output sensor file.  Default: /tmp/sensors/mem.txt
+  -t TEMP_DIR    temporary directory. Default: /tmp
+```