Automated nightly builds

[nnansters]

Added a workflow definition that automatically builds a new PDF version when pushing to main or on PR.

The resulting PDF is uploaded to a static "pre-release" so we can have a single stable URL to link to in the README file afterwards.

Note:

• this will require us to manually create the "nightly" release first.
• not sure if a new workflow token will have to be set up (I did do it in my fork first)