Add check for identical duplicate entries in global navigation

What does this MR do and why?

Because duplicate navigation items don't behave correctly, this MR:

• Fixes instances of identical duplicate identical navigation items.
• Adds a shell script to check for future identical duplicate navigation items.
• Adds shell script to:
  • Lefthook configuration so that folks that have identical duplicated navigation items can catch them before kicking off a pipeline.
  • Adds shells script to Makefile, so it can be run with make test.
  • Run make target in pipelines, to eliminate regressions.

This check doesn't find instances of duplicate navigation items that are differentiated using a trailing anchor tag.

How to set up and validate locally

1. Configure a local GitLab Docs environment: https://gitlab.com/gitlab-org/gitlab-docs/-/blob/main/doc/setup.md.
2. Check out this branch.
3. Run make check-global-navigation. Test passes.
4. Duplicate a navigation item in content/_data/navigation.yaml.
5. Run make check-global-navigation. Test fails.

Merge request acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this merge request.