Updated read the docs documentation to include cache operation.

Author: david-sh-csiro

docs/api/operations/cache.rst

@@ -0,0 +1,10 @@
+=========================
+emsarray.operations.cache
+=========================
+
+.. automodule:: emsarray.operations.cache
+
+Functions
+=========
+
+.. autofunction:: make_cache_key

docs/api/operations/index.rst

@@ -13,6 +13,10 @@ and behave the same across all supported conventions.
 
    ./*
 
+:doc:`cache`
+    These operations create hash keys of datasets for use
+    in caching data.
+
 :doc:`depth`
     These operations manipulate datasets with a depth axis,
     such as the output of ocean models.