Add check for index.html to global navigation check script

What does this MR do and why?

Following on from: !2993 (merged), we can reduce the number of jobs we have and check for index.html in the global navigation in more places.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Configure a local GitLab Docs environment: https://gitlab.com/gitlab-org/gitlab-docs/-/blob/main/doc/setup.md.
2. Run make check-global-navigation. Tests pass.
3. Add index.html to the end of an entry in content/_data/navigation.yaml. 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.