docs/server: Fixed PDF generation

ReadTheDocs is not supported SVG images in PDF docs.
readthedocs/readthedocs.org#905

Author: andras-tim

docs/_static/model/sql_model.png

@@ -3,7 +3,7 @@
 SQL Model
 =========
 
-.. image:: ../_static/model/sql_model.svg
+.. image:: ../_static/model/sql_model.*
    :width: 636px
    :height: 590px
    :alt: SQL Model
@@ -12,7 +12,7 @@ SQL Model
 Model with indexes
 ------------------
 
-.. image:: ../_static/model/sql_model_with_indexes.svg
+.. image:: ../_static/model/sql_model_with_indexes.*
    :width: 636px
    :height: 699px
    :alt: SQL Model with indexes