check-pages-not-in-nav: Add more exceptions for directories

What does this MR do and why?

Adds more exceptions to the check-pages-not-in-nav command so the results are more actionable.

• Related to Pages not in the nav, March 2024 (#1776 - closed)
• Related to Docs project maintenance tasks, March 2024 (technical-writing#1006 - closed)

How to set up and validate locally

Run this command in the root of the gitlab-docs repo:

make check-pages-not-in-nav | jq '.[] | .url' > pages_not_in_nav.txt

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.

Closes #1776 (closed)