Bump Markdig from 0.39.1 to 0.40.0 (#924)

Bumps [Markdig](https://github.com/xoofx/markdig) from 0.39.1 to 0.40.0.
- [Release notes](https://github.com/xoofx/markdig/releases)
- [Changelog](https://github.com/xoofx/markdig/blob/master/changelog.md)
- [Commits](xoofx/markdig@0.39.1...0.40.0)

---
updated-dependencies:
- dependency-name: Markdig
  dependency-version: 0.40.0
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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Author: dependabot[bot]