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)
