clean-dialect-docs should remove the entire dialect docs build

[paul0403]

Context:
make clean-dialect-docs should remove the entire dialect docs build directory, instead of just a subdirectory inside.

Benefits:
Cleaning works.
Otherwise, the cmake files from the previous builds are still in the build directory.

[github-actions]

Hello. You may have forgotten to update the changelog!
Please edit doc/releases/changelog-dev.md on your branch with:

• A one-to-two sentence description of the change. You may include a small working example for new features.
• A link back to this PR.
• Your name (or GitHub username) in the contributors section.

[rniczh]

Thanks @paul0403 👍