Bump babel from 2.9.0 to 2.9.1 in /docs

Bumps [babel](https://github.com/python-babel/babel) from 2.9.0 to 2.9.1.
- [Release notes](https://github.com/python-babel/babel/releases)
- [Changelog](https://github.com/python-babel/babel/blob/master/CHANGES)
- [Commits](python-babel/babel@v2.9.0...v2.9.1)

---
updated-dependencies:
- dependency-name: babel
  dependency-type: indirect
...

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

Author: dependabot[bot]