update doc

Author: b8raoult

docs/building/introduction.rst

@@ -14,7 +14,7 @@ computed as the dataset is build, and stored in the metadata, with other
 information such as the the locations of the grid points, the list of
 variables, etc.
 
-.. figure:: ../schemas/recipe.svg
+.. figure:: ../schemas/recipe.png
    :alt: Building datasets
    :align: center
 

docs/overview.rst

@@ -24,7 +24,10 @@ ensemble, and fourth dimension are the grid points values.
  Chunking
 **********
 
-.. figure:: schemas/matrix.svg
+By default, the array is chunked along the time dimensions so the whole
+state of the atmosphere at a given time is loaded in one go:
+
+.. figure:: schemas/matrix.png
    :alt: Data chunking
    :align: center