Add PDF export to pkgdocs deployment

[ickc]

The Pkg documentation at https://pkgdocs.julialang.org/v1/ currently lacks a PDF download option, unlike the main Julia documentation at https://docs.julialang.org/en/v1/ which provides one.

I noticed there's already a PDF target in the project's build configuration, but it's not being deployed to gh-pages alongside the HTML version. Adding the PDF to the deployment would provide:

• Offline access - Read Pkg documentation without internet connectivity
• Better study workflow - Add highlights, annotations, and bookmarks directly in the document
• Research integration - Import into reference managers like Zotero for easier citation and retrieval
• Consistency - Match the main Julia documentation's offering of multiple formats

Since the PDF target already exists, this should just require adding it to the gh-pages deployment process. Would you be open to including the built PDF in the deployment?

[ickc]

Actually, pdf are already built and uploaded in https://github.com/JuliaLang/Pkg.jl/blob/gh-pages-pdf/v1.11.6/Pkg.jl.pdf
I guess the only remaining issue is that it is not linked from the documentation site?