Fix/GH-787: Copy downloads directory into _site on build

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Checklist

Please put an X within the brackets that apply [X].

[X] I have read the CONTRIBUTING.md file
[X] I have made my changes in a new branch and not directly in the main branch
[X] This pull request is ready for final review by the Quire team

Is this pull request related to an open issue? If so, what is the issue number?

Issue #787

Please briefly describe the goal of this pull request and how it may impact Quire's functionality.

After outputting PDF and EPUB files, Quire users most often make those available for download from the deployed project site. Our practice for this is to add them to an _assets/downloads/ directory. This PR ensures that directory is copied over to the _site directory when running quire build.

Please describe the changes you made, and call out any details you think are particularly relevant for the Quire team to note in their review.

Updates Vite configuration to copy the downloads directory, along with the images and fonts directories we were already copying over.

Does this pull request necessitate changes to Quire's documentation?

The PDF and EPUB instructions could be clarified a little. I'll submit that as a PR to the quire-docs site shortly.

thegetty / quire