fix: filter out build dir from Vale checks

[dwilding]

This PR fixes the following bug:

1. Clone a repo with documentation, e.g., https://github.com/canonical/sphinx-stack-docs
2. Run make -C docs html
3. Run make -C docs spelling

The spelling check fails on the LLM .md files in _build because those .md files don't have the rich formatting that Vale would normally ignore.

---

• Have you updated CHANGELOG.md with relevant non-documentation file changes?
• Have you updated the documentation for this change?

[dwilding]

@jahn-junior does this look reasonable to you?

I haven't updated CHANGELOG.md yet because it's still in a pre-2.0 state. I thought I'd better check with you first before modifying it (or opening a PR to sync with 2.0).

[jahn-junior]

@dwilding

@jahn-junior does this look reasonable to you?

Yes! I made the same patch when I was testing sphinx-llm, but I must've forgotten to add it to the final implementation. Thanks for the fix!

I haven't updated CHANGELOG.md yet because it's still in a pre-2.0 state. I thought I'd better check with you first before modifying it (or opening a PR to sync with 2.0).

That's my bad. If you're fine with the out-of-scope change, you're welcome to change the existing header to "2.0" and add a new "Upcoming" section. I'm not super picky about the formatting since the changelog is mainly consumed by the update script.