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/buffer/buffer.h

@@ -31,12 +31,17 @@ extern "C" {
 #include <spa/utils/defs.h>
 #include <spa/buffer/meta.h>
 
-/** \page page_buffer Buffers
+/** \defgroup spa_buffer SPA Buffers
  *
  * Buffers describe the data and metadata that is exchanged between
  * ports of a node.
  */
 
+/**
+ * \addtogroup spa_buffer
+ * \{
+ */
+
 enum spa_data_type {
 	SPA_DATA_Invalid,
 	SPA_DATA_MemPtr,		/**< pointer to memory, the data field in
@@ -112,6 +117,10 @@ static inline void *spa_buffer_find_meta_data(const struct spa_buffer *b, uint32
 	return NULL;
 }
 
+/**
+ * \}
+ */
+
 #ifdef __cplusplus
 }  /* extern "C" */
 #endif