docs: add link to complete list of categories (#2934)

When user read about these anotation +@complete -@polyfill, they wonder where it came from.

It is not immediately obvious where to see it without reading the next section.

Author: kocoten1992

docs/tutorials/plugins.md

@@ -108,6 +108,7 @@ python build/build.py +@complete -@polyfill
 python build/build.py +@complete -@polyfill -@text
 ```
 
+To see the complete list of categories, its in [`build/types/`](https://github.com/google/shaka-player/tree/master/build/types)
 
 #### Build Configs