DOC: Add alt text to images

src/doc/converters/index.rst

@@ -7,6 +7,8 @@ existing StructureElements, Converters create a tree of StructureElements.
 
 .. image:: ../img/converter.png
   :height: 170
+  :alt: Converters are Python classes that tell the crawler how to
+        interprete StructureElements.
 
 Each StructureElement in the tree has a set of properties, organized as
 key-value pairs.

src/doc/converters/standard_converters.rst

@@ -182,6 +182,8 @@ with value "Silvia Scientist":
 
 .. image:: ../img/properties-from-dict-records-author.png
   :height: 210
+  :alt: A Record "New Name" and an author Record with full_name
+        "Silvia Scientist" are generated and filled automatically.
 
 Note how the different dictionary keys are handled differently
 depending on their types: scalar and list values are understood
@@ -216,6 +218,8 @@ property:
 
 .. image:: ../img/properties-from-dict-records-person.png
   :height: 200
+  :alt: A new Person Record is created which is referenced as an
+        author.
 
 For the time being, only the parents of the referenced record can be
 set via this option. More complicated treatments can be implemented