fix: replace deprecated sphinx default (#3512)

Removes a warning from doc build

doc/conf.py

@@ -60,7 +60,7 @@
 # The suffix(es) of source filenames.
 # You can specify multiple suffix as a list of string:
 # source_suffix = ['.rst', '.md']
-source_suffix = ".rst"
+source_suffix = {".rst": "restructuredtext"}
 
 # The encoding of source files.
 # source_encoding = 'utf-8-sig'