Bump @backstage/plugin-techdocs from 1.8.0 to 1.9.1 in /backstage

Bumps [@backstage/plugin-techdocs](https://github.com/backstage/backstage/tree/HEAD/plugins/techdocs) from 1.8.0 to 1.9.1.
- [Release notes](https://github.com/backstage/backstage/releases)
- [Changelog](https://github.com/backstage/backstage/blob/master/plugins/techdocs/CHANGELOG.md)
- [Commits](https://github.com/backstage/backstage/commits/v1.9.1/plugins/techdocs)

---
updated-dependencies:
- dependency-name: "@backstage/plugin-techdocs"
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>