Investigate broken documentation CI

[loostrum]

Documentation fails to build due to an issue in exhale.
See e.g. https://github.com/nlesc-recruit/PowerSensor3/actions/runs/6614710010/job/17965260946#step:4:18

Run make -C docs html && make -C docs html
make: Entering directory '/home/runner/work/PowerSensor3/PowerSensor3/docs'
Running Sphinx v7.2.6
/home/runner/.local/lib/python3.10/site-packages/breathe/project.py:116: RemovedInSphinx80Warning: Sphinx 8 will drop support for representing paths as strings. Use "pathlib.Path" or "os.fspath" instead.
  self._default_build_dir = os.path.dirname(app.doctreedir.rstrip(os.sep))
WARNING: html_static_path entry '_static' does not exist
making output directory... done
/home/runner/.local/lib/python3.10/site-packages/exhale/configs.py:1408: RemovedInSphinx80Warning: Sphinx 8 will drop support for representing paths as strings. Use "pathlib.Path" or "os.fspath" instead.
  _one     = containmentFolder == app.srcdir

Extension error (exhale):
Handler <function environment_ready at 0x7f11bcc1cd30> for event 'builder-inited' threw an exception (exception: startswith first arg must be str or a tuple of str, not _StrPath)
make: *** [Makefile:20: html] Error 2
make: Leaving directory '/home/runner/work/PowerSensor3/PowerSensor3/docs'
Error: Process completed with exit code 2.