Support webpdf format

nbconvert v6.0 introduced a new `webpdf` format that generates a PDF via HTML rather than via LaTeX.

https://nbconvert.readthedocs.io/en/latest/usage.html#convert-webpdf

This may need some work on dependencies for CI, since it requires a headless Chromium browser. The documentation suggests that simply installing `pyppeteer` might be enough.

Start by reading nbautoexport’s existing format and export handling, then review the linked nbconvert webpdf documentation. Check the project’s CI dependency setup and verify whether pyppeteer provides the required headless Chromium support. Done means nbautoexport can export notebooks in webpdf format and the relevant CI checks pass.

Written by the indexing model from the issue text.