Fix error during linkcheck when trying to write pygments.css

[s-weigand]

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

However, our CI fails during lickcheck since the parent folder where the theme tries to write pygments.css to does not exist during link checking.

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 in our conf.py which lets the CI pass again.

This error didn't surface here for 2 reasons:

• Linkchecking detects broken links, causing the function to exit early
• The HTML build folder is passed to linkcheck