ci: remove unused --parents option

https://ss64.com/bash/mkdir.html
We would want to see an error if `doc/` didn't exist. The docs wouldn't build as all the content is in that directory.

Working on migrating to a cross-platform shell that doesn't require this option anyway https://www.nushell.sh/commands/docs/mkdir.html

pyproject.toml

@@ -179,7 +179,7 @@ clean-all-win = [
     "if exist doc\\_images rd /s /q doc\\_images",
 ]
 build-html = [
-    "mkdir -p doc/_images",
+    "mkdir doc/_images",
     "sphinx-build -b html -d doc/_build/doctrees doc doc/_build/html",
 ]
 build-html-win = [