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/graph/graph.h

@@ -29,6 +29,14 @@
 extern "C" {
 #endif
 
+/** \defgroup spa_graph SPA Graph
+ */
+
+/**
+ * \addtogroup spa_graph
+ * \{
+ */
+
 #include <spa/utils/defs.h>
 #include <spa/utils/list.h>
 #include <spa/utils/hook.h>
@@ -345,6 +353,10 @@ static const struct spa_graph_node_callbacks spa_graph_node_impl_default = {
 	.reuse_buffer = spa_graph_node_impl_reuse_buffer,
 };
 
+/**
+ * \}
+ */
+
 #ifdef __cplusplus
 }  /* extern "C" */
 #endif