Bump zensical from 0.0.20 to 0.0.21 in /docs

Bumps [zensical](https://github.com/zensical/zensical) from 0.0.20 to 0.0.21.
- [Release notes](https://github.com/zensical/zensical/releases)
- [Commits](zensical/zensical@v0.0.20...v0.0.21)

---
updated-dependencies:
- dependency-name: zensical
  dependency-version: 0.0.21
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]