disable smartquotes

Smartquotes will convert '"..."' to '“...”', will cause unexpected
result when we reference a string without using code block.

also in some languages (zh_TW for example), it will be converted to
'「...」', usually not a desired result.

conf.py

@@ -101,6 +101,8 @@ from sphinx.highlighting import lexers
 lexers["gdscript"] = GDScriptLexer()
 # fmt: on
 
+smartquotes = False
+
 # Pygments (syntax highlighting) style to use
 pygments_style = "sphinx"
 highlight_language = "gdscript"