Bump jupyter-book from 0.13.1 to 0.13.2 in /docs

Bumps [jupyter-book](https://github.com/executablebooks/jupyter-book) from 0.13.1 to 0.13.2.
- [Release notes](https://github.com/executablebooks/jupyter-book/releases)
- [Changelog](https://github.com/executablebooks/jupyter-book/blob/master/CHANGELOG.md)
- [Commits](jupyter-book/jupyter-book@v0.13.1...v0.13.2)

---
updated-dependencies:
- dependency-name: jupyter-book
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

Author: dependabot[bot]

docs/requirements.txt

@@ -1 +1 @@
-jupyter-book==0.13.1
+jupyter-book==0.13.2