Fix error during linkcheck when trying to write pygments.css (#2097)

Hi there, I'm currently changing our docs to use your nice theme 😄 

However, [our CI fails during
lickcheck](https://github.com/s-weigand/pyglotaran-extras/actions/runs/12713637790/job/35441962465)
since the parent folder where the theme tries to write `pygments.css` to
does not exist during link checking.

```py
Extension error (pydata_sphinx_theme.pygments):
Handler <function overwrite_pygments_css at 0x7f93feb0e4d0> for event 'build-finished' threw an exception (exception: [Errno 2] No such file or directory: '/home/runner/work/pyglotaran-extras/pyglotaran-extras/docs/_build/linkcheck/_static/pygments.css')
make: *** [Makefile:25: linkcheck] Error 2
make: Leaving directory '/home/runner/work/pyglotaran-extras/pyglotaran-extras/docs'
```

My workaround on our side is to [create the expected parent path for
linkcheck](https://github.com/s-weigand/pyglotaran-extras/blob/f5b35f98e63e68dee73b347f175ee6c7abfe35f0/docs/conf.py#L27-L29)
in our `conf.py` which [lets the CI pass
again](https://github.com/s-weigand/pyglotaran-extras/actions/runs/12844533235/job/35817635012).

This error didn't surface here for 2 reasons:
- Linkchecking detects broken links, causing the function to [exit
early](https://github.com/pydata/pydata-sphinx-theme/blob/d76892d437e3c0dc8a059741a84afbd6cc458509/src/pydata_sphinx_theme/pygments.py#L63)
- The [HTML build folder is passed to
linkcheck](https://github.com/pydata/pydata-sphinx-theme/blob/d76892d437e3c0dc8a059741a84afbd6cc458509/tox.ini#L44)

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Tania Allard <[email protected]>

Author: 3 people

src/pydata_sphinx_theme/pygments.py

@@ -99,5 +99,6 @@ def overwrite_pygments_css(app: Sphinx, exception=None):
 
     # re-write pygments.css
     pygments_css = Path(app.builder.outdir) / "_static" / "pygments.css"
-    with pygments_css.open("w") as f:
-        f.write(get_pygments_stylesheet(light_theme, dark_theme))
+    # Ensure the _static folder exists for all builders
+    pygments_css.parent.mkdir(exist_ok=True)
+    pygments_css.write_text(get_pygments_stylesheet(light_theme, dark_theme))