spa: group the spa headers into doxygen modules

Subdirectories buffer, control, debug, monitor, pod, support and utils, others
are still missing. Headers are grouped either per subdirectory (e.g. buffer/
gets added to group spa_buffer) or per-file (e.g. spa_json is a separate
group), whatever seemed like the most sensible approach.

spa/include/spa/param/format.h

@@ -29,6 +29,11 @@
 extern "C" {
 #endif
 
+/**
+ * \addtogroup spa_param
+ * \{
+ */
+
 #include <spa/param/param.h>
 
 /** media type for SPA_TYPE_OBJECT_Format */
@@ -140,6 +145,10 @@ enum spa_format {
 #define SPA_KEY_FORMAT_DSP		"format.dsp"		/**< a predefined DSP format,
 								  *  Ex. "32 bit float mono audio" */
 
+/**
+ * \}
+ */
+
 #ifdef __cplusplus
 }  /* extern "C" */
 #endif