`check-removed-urls.yml` is too strict

Today I noticed that some include files get rendered as full pages in our documentation.
https://github.com/canonical/launchpad-manual/tree/main/docs/includes

When I added our `includes` folder to the excludes configuration in `conf.py`, the rendering was fixed, but CI broke, because those pages are no longer rendered.
https://github.com/canonical/launchpad-manual/pull/438

```
Run BASE_URLS_PATH="base/docs/urls.txt"
The following URLs were removed:
./includes/important_not_revised/
./includes/important_not_revised_help/
./includes/important_out_of_date/
Please ensure removed pages are redirected
Error: Process completed with exit code 1.
```

This change is intentional, but CI is red.

It was suggested that I should merge my changes anyway, but encouraging users to merge changes with a broken CI is a very slippery slope which I do not endorse.

I think the issue is that there is no way to tell the script that this is intentional.
https://github.com/canonical/sphinx-docs-starter-pack/blob/main/.github/workflows/check-removed-urls.yml

GitHub comes with a `skip CI` directive (https://docs.github.com/en/actions/how-tos/manage-workflow-runs/skip-workflow-runs), but this is a too big hammer, as then other checks would be skipped, too.

There is a way to skip individual workflows. I will propose a PR soon.