Replace single quotes with double quotes

This MR replaces all single quotes with double quotes (where appropriate), consistency is nice.

This was applied automatically via

ruff check --select Q --fix .

docs/conf.py

@@ -14,14 +14,14 @@ project = "gwdatafind"
 copyright = "2018-2025, Cardiff University"
 author = "Duncan Macleod"
 release = gwdatafind.__version__
-version = re.split(r'[\w-]', gwdatafind.__version__)[0]
+version = re.split(r"[\w-]", gwdatafind.__version__)[0]
 
 # -- config
 
-source_suffix = '.rst'
-master_doc = 'index'
+source_suffix = ".rst"
+master_doc = "index"
 
-default_role = 'obj'
+default_role = "obj"
 
 # -- theme