Add a script to report on pages not in the global nav

What does this MR do and why?

Closes #1212 (closed)

Adds a script to list pages that are not included in the site's global navigation and require manual review.

This script can be used by the TW team as a part of the monthly maintenance task list.

How to set up and validate locally

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

1. Check out this branch on your local copy of the site: git fetch --all && git checkout sarahg/1212-pages-no-nav
2. If you're spot-checking results, make sure all your content repos are up-to-date: make update-all-docs-projects
3. Run the script with a make command: make check-pages-not-in-nav

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.