Sensitivity to absolute vs. relative paths in artifacts.toml

[bradcray]

While looking at Google Search Console errors for our website, I noticed a few issues:

• if an entry in artifacts.toml is cross-listed on multiple pages (say, presentations and resources), it needs to have an absolute (hugo) path like /presentations/mySlides.pdf rather than simply mySlides.pdf
• similarly, any entry that shows up in the top-level index.xml (which seems to be the initial entries on any given page) needs to use a similar absolute path in order to show up accurately there

This makes me think that we should probably require all artifacts.toml entries to use absolute (hugo) paths rather than relative ones, to avoid introducing errors. Or incorporate a link checker somewhere into our chapel-www workflow to catch these cases before Google Search Console does(#49).