Docs for API and schedule mixins

docs/extend/plugins/api.md

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+---
+title: Schedule Mixin
+---
+
+## APICallMixin
+
+The APICallMixin class provides basic functionality for integration with an external API.

docs/extend/plugins/schedule.md

@@ -1,3 +1,7 @@
 ---
 title: Schedule Mixin
 ---
+
+## ScheduleMixin
+
+The ScheduleMixin class provides a plugin with the ability to call functions at regular intervals.