Adding github workflow to generate pdf

[duffee]

This addresses #2

I've tried to make a bare-bones workflow that uses few resources. It will trigger on a push to the master branch or on a manual trigger in the Github Actions menu. The run took 3m 40s. Because retaining artifacts can have an impact on organization costs, I've dropped the retention from 90 days default to 3 days. You can see the process log and artifact produced (this one retained 5 days) at https://github.com/duffee/pdl-book/actions/runs/26768959131/job/78902919258

PDL-Book is a zip file that contains PDL_Book.pdf and looks how I'd expect.

This is my first workflow, so comments welcome, especially if there is a lighter version of pdfjam. I'm installing the perl dependencies after the apt-get update/install so that I can say --notest, but the commented line in Setup Perl would do the same thing.

[duffee]

For the curious, I have another workflow using perl-actions/install-with-cpanm instead in the generate_pdf_actions_experimental branch. Not much difference in the execution time (3m 19s, a little longer setting up Perl, but saves time in writing the pdf. probably not significant)

[duffee]

Closes #2

I've added a condition on the push to only generate a pdf when something under the PDL/ directory changes, meaning only textual changes trigger the action. It adds a cpanfile to handle installing the dependencies with cpanm during the Setup Perl stage.