add why trees

Author: ianhi

_toc.yml

@@ -46,6 +46,7 @@ parts:
         sections:
           - file: intermediate/indexing/advanced-indexing.ipynb
           - file: intermediate/indexing/boolean-masking-indexing.ipynb
+          - file: intermediate/indexing/why-trees.md
       - file: intermediate/xarray_and_dask
       - file: intermediate/intro-to-zarr.ipynb
       - file: intermediate/storage_formats.ipynb