[ADD] conf: intersphinx entry for pillow

Allows us to properly link to pillow's docs from docstrings using the
python domain.

Part-of: odoo/documentation#1352

conf.py

@@ -137,6 +137,7 @@ if odoo_dir_in_path:
 todo_include_todos = False
 
 intersphinx_mapping = {
+    'pillow': ('https://pillow.readthedocs.io/en/stable/', None),
     'python': ('https://docs.python.org/3/', None),
     'werkzeug': ('https://werkzeug.palletsprojects.com/en/1.0.x/', None),
 }