fix: Add quiet flag to linkchecker

[nhennigan]

It was reported in canonical/documentation-workflows#37 issue that the verbose output of all the OK links in the make linkchecker command made it difficult to see actual errors. Adding the -q flag to the linkcheck command will mean only errors and broken links will be printed.

This will be picked up by the workflows too as they use the Makefile in the repo they are running on to decide what parameters to feed the linkchecker.

• Have you updated CHANGELOG.md with relevant non-documentation file changes?
  Can I get some guidance on how the changelog should look?
• Have you updated the documentation for this change?
  The documentation on the linkchecker does not go into this level of detail and does not need to be updated

---

[medubelko]

@nhennigan The workflow for adding to the changelog is a bit odd. It's self-referential. After creation of the PR, go back to your branch and add another commit that modifies the changelog and includes a link to the PR itself.

Since there's no changelog entry for the next version yet, here's what I'd add to the start of the log:

## Upcoming

* <brief description>

### Changed

* `docs/Makefile` [#536](https://github.com/canonical/sphinx-docs-starter-pack/pull/536)

[nhennigan]

Thanks @medubelko I was unsure of what title to add as I am not creating a release based solely on this fix. I have added it now and rebased to main so I get the broken links fixes.

[medubelko]

🚀 https://github.com/canonical/sphinx-docs-starter-pack/actions/runs/22205199634/job/64227419981?pr=536#step:4:246

[jahn-junior]

LGTM. Thanks!