chore: add tw-docops CODEOWNERS for docs lint config

Summary

Gives @gitlab-org/technical-writing/tw-docops ownership of the Vale config (docs/.vale/) and the markdownlint config (.markdownlint-cli2.yaml), matching the pattern already used in gitlab-org/gitlab.

• Related to Update CODEOWNERS across all docs projects (gitlab-org/technical-writing/docs-gitlab-com#758)

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