build JOSS pdf on push

[orbeckst]

We can use the action (with modification) https://github.com/Becksteinlab/zarrtraj/blob/main/.github/workflows/draft-pdf.yml to build the PDF as an artifact.

[orbeckst]

Seems to work, see https://github.com/MDAnalysis/parallelization-benchmarks/actions/runs/20251519685?pr=4 and download paper.zip, unpack and look at paper.pdf