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

(cherry picked from commit 2636f50dd4)

doc/manual/substitute.py

@@ -41,6 +41,10 @@ def recursive_replace(data: dict[str, t.Any], book_root: Path, search_path: Path
             return data | dict(
                 sections = [recursive_replace(section, book_root, search_path) for section in sections],
             )
+        case {'items': items}:
+            return data | dict(
+                items = [recursive_replace(item, book_root, search_path) for item in items],
+            )
         case {'Chapter': chapter}:
             path_to_chapter = Path(chapter['path'])
             chapter_content = chapter['content']