Missing and broken links in contributing pages

[brihall]

In content/contributing/index.md, there are several guides without links and labeled as "Coming Soon". It seems like a few of those same guides are linked in other contributing pages though.

• setup.md in quick.md (line 5) and pr.md (line 12)
• tests.md in pr.md (line 52)

There are also a handful of broken links in index.md and pr.md that I can submit a PR to fix. It's not clear to me though whether the missing links on the index page should be updated, because they were removed from index.md and added to the other pages in the same commit.