doc: add configurations Doxygen group

doc.txt

+/**
+ * @defgroup config Compile time configurations
+ * @brief    List of compile-time configurations
+ *
+ * This group contains all compile-time configurations macros that are
+ * intended to be modified by the user.
+ *
+ * These configuration macros are defined within modules like this:
+ * ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ [.c]
+ * #ifndef SOME_CONFIGURATION_PARAM
+ * #define SOME_CONFIGURATION_PARAM DEFAULT_VALUE
+ * #endif
+ * ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ * Because of that, configurations can be altered e.g via CFLAGS (e.g
+ * `CFLAGS += -DSOME_CONFIGURATION_PARAM=SOME_VALUE`)
+ */
+
 /**
  * @defgroup utils Utilities
  * @brief Utilities and helper functionality