Add why-an-index to TOC and link it from the SciPy 2026 outline (#370)

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-an-index.ipynb
       - file: intermediate/xarray_and_dask
       - file: intermediate/intro-to-zarr.ipynb
       - file: intermediate/storage_formats.ipynb

workshops/scipy2026/index.ipynb

@@ -52,7 +52,7 @@
     "| *10 minute Break* |\n",
     "| Data cataloguing | 3:50 (40 min) | [Data cataloging](../../intermediate/cataloging.ipynb) |\n",
     "| *10 minute Break* \n",
-    "| Flexible indices | 4:40 (50 min) | Why an index |"
+    "| Flexible indices | 4:40 (50 min) | [Why an index](../../intermediate/indexing/why-an-index.ipynb) |"
    ]
   },
   {