MAINT: Reenable parallel write safety (#1859)

See scipy/scipy#20897. TL;DR is that the problem
was always with reading in parallel, not writing. And the bug is really
in Sphinx.

It would be good to detect and clean this up in PST somehow perhaps as
well -- maybe with a warning if a top-level / root page ever shows up as
a child? This *might* only be an issue when building in parallel, still
not totally clear to me why things were okay in the serial case.

I think any additional changes could be made in a follow-up PR probably.

closes #1643

src/pydata_sphinx_theme/__init__.py

@@ -291,4 +291,4 @@ def setup(app: Sphinx) -> Dict[str, str]:
     # Include component templates
     app.config.templates_path.append(str(theme_path / "components"))
 
-    return {"parallel_read_safe": True, "parallel_write_safe": False}
+    return {"parallel_read_safe": True, "parallel_write_safe": True}