doc: fix "edit source" widget (#1597)

Without the submodules and separate builds, there is now a simple fix
for the edit source widget.

fixes dylan-lang/website#148

documentation/source/conf.py

@@ -45,7 +45,7 @@
     # https://pradyunsg.me/furo/customisation/edit-button/
     'source_repository': 'https://github.com/dylan-lang/opendylan',
     'source_branch': 'master',
-    'source_directory': 'source/',
+    'source_directory': 'documentation/source',
 }
 html_theme_path = ['_themes']   # still needed? add furo submodule here?
 html_title = 'Open Dylan'