Bump linuxdoc from 20210324 to 20211220 (#3135)

Bumps [linuxdoc](https://github.com/return42/linuxdoc) from 20210324 to 20211220.
- [Release notes](https://github.com/return42/linuxdoc/releases)
- [Commits](https://github.com/return42/linuxdoc/commits

)

---
updated-dependencies:
- dependency-name: linuxdoc
  dependency-type: direct:development
  update-type: version-update:semver-major
...

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Noémi Ványi <kvch@users.noreply.github.com>