build(deps): update jupyter-book requirement

Updates the requirements on [jupyter-book](https://github.com/jupyter-book/jupyter-book) to permit the latest version.
- [Release notes](https://github.com/jupyter-book/jupyter-book/releases)
- [Commits](jupyter-book/jupyter-book@v0.15.0...v2.1.0)

---
updated-dependencies:
- dependency-name: jupyter-book
  dependency-version: 2.1.0
  dependency-type: direct:production
...

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

Author: dependabot[bot]

requirements-docs.txt

@@ -1,3 +1,3 @@
 # For documentation
 ghp-import>=2.1
-jupyter-book>=0.15,<2.0
+jupyter-book>=0.15,<3.0