Enable linkchecker on books

Previously, mdBook used JavaScript to add header links, so we
skipped checking the book. As of
https://github.com/rust-lang/rust/pull/39966, it no longer does,
so we can start checking again.

There is a twist, though: it uses name instead of id, so let's test
for both. They're both valid links anyway, so it's good to have the
checker check anyway.

0 files changed, 0 insertions, 0 deletions