maint: Remove mdbook-linkcheck and support mdbook 0.5.x

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

doc/manual/package.nix

@@ -6,7 +6,6 @@
   ninja,
   lowdown-unsandboxed,
   mdbook,
-  mdbook-linkcheck,
   jq,
   python3,
   rsync,
@@ -51,7 +50,6 @@ mkMesonDerivation (finalAttrs: {
     ninja
     (lib.getBin lowdown-unsandboxed)
     mdbook
-    mdbook-linkcheck
     jq
     python3
     rsync