docs: Add version added to filter index functions

These functions were added with OBS 30.0.0

docs/sphinx/reference-sources.rst

@@ -1732,6 +1732,8 @@ Filters
 
    :param index: | The index to move the filter to.
 
+   .. versionadded:: 30.0
+
 ---------------------
 
 .. function:: int obs_source_filter_get_index(obs_source_t *source, obs_source_t *filter)
@@ -1740,6 +1742,8 @@ Filters
 
    :return: Index of the filter or -1 if it is invalid/not found.
 
+   .. versionadded:: 30.0
+
 Functions used by filters
 -------------------------