verify-links: catch broken anchors, directory links, and invisible characters (#33713)

* verify-links: catch broken anchors, directory links, and invisible characters

Enhances the link verification script to catch several categories of
broken links that were previously missed:

- Broken anchors (cross-file and same-file) by extracting heading slugs
  from target documents and verifying anchors resolve
- Directory links missing index.md suffix within docs/
- Invisible/zero-width characters in URLs
- Case-sensitive anchor mismatches

Also strips fenced code blocks before scanning for links to avoid false
positives, and handles duplicate heading slug deduplication (GitHub and
Docusaurus append -1, -2, etc.).

Fixes a few newly-caught broken links in existing docs.

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>
Made-with: Cursor

* Fix CodeQL incomplete multi-character sanitization alert

Apply HTML tag stripping in a loop so that nested fragments
like <scr<script>ipt> are fully removed.

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>
Made-with: Cursor

---------

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>

REVIEWING.md

@@ -143,7 +143,7 @@ Some things that changeset should NOT contain are:
 
 ### Backstage UI Changeset Format
 
-Changesets for `@backstage/ui` must follow a standardized format to enable proper documentation generation. See [`.changeset/README.md`](.changeset/README.md#backstage-ui-changeset-format) for the complete guide.
+Changesets for `@backstage/ui` must follow a standardized format to enable proper documentation generation. See [`.changeset/README.md`](.changeset/README.md#backstage-ui-changesets) for the complete guide.
 
 **Required structure:**