Fixes#14628
- Remove mdbook-linkcheck dependency and configuration (was blocking
upgrades to mdbook 0.5.0+, configured with warning-policy = "ignore"
due to false positives, and redundant with lychee-based link checking)
- Update substitute.py and anchors.jq to handle 'items' (mdbook 0.5.x)
in addition to 'sections' (mdbook 0.4.x), as per mdbook 0.5.0
changelog: "Book::sections was renamed to Book::items"
https://github.com/rust-lang/mdBook/blob/master/CHANGELOG.md#05-migration-guide
(cherry picked from commit 2636f50dd4)
Python is only pulled into the build closure by Mercurial, which might end up being removed.
Let’s port the script to jq, which is more likely to stay.
