Add support for ignored pages in navigation report

What does this MR do and why?

To solve #1794 (closed), let's be able to explicitly mark some pages as "ignorable" for the not-in-global-nav report, so that folks doing the monthly chores don't need discover the context for that should be ignored.

This adds to the existing exclusion system (https://gitlab.com/gitlab-org/gitlab-docs/-/blob/21e1e04282c359c804e4504da03fa2522ba8c84d/scripts/pages_not_in_nav.js#L60), but is more visible. The existing system is better for excluding whole directories. This system is better for single pages.

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. Check out this branch.
3. Run make check-pages-not-in-nav and see report.
4. Run VERBOSE=true make check-pages-not-in-nav and see verbose report.
5. Update a docs page to have ignore_in_report: true in the metadata.
6. Run VERBOSE=true make check-pages-not-in-nav. See the ignored page listed in verbose report.

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.