Make guides and tutorials citable with Zenodo

[jeffjennings]

Implementation of decision in astropy-tutorials discussion

• Confirm authorship for citation purposes
• Add github action to astropy-tutorials to capture generated PDFs of all guides/tutorials using artifact generated by upload-artifact step of action
• Upload PDFs of all current guides/tutorials to Zenodo (use a GitHub action that automates this)
• Add 'Citing this work' section to each notebook on astropy-tutorials
• Update website guidance on notebook format with this section
• Decide on how frequently to update Zenodo records