Bump quantecon-book-theme from 0.9.2 to 0.15.0

Bumps [quantecon-book-theme](https://github.com/QuantEcon/quantecon-book-theme) from 0.9.2 to 0.15.0.
- [Release notes](https://github.com/QuantEcon/quantecon-book-theme/releases)
- [Changelog](https://github.com/QuantEcon/quantecon-book-theme/blob/main/CHANGELOG.md)
- [Commits](QuantEcon/quantecon-book-theme@v0.9.2...v0.15.0)

---
updated-dependencies:
- dependency-name: quantecon-book-theme
  dependency-version: 0.15.0
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

Author: dependabot[bot]

environment.yml

@@ -7,7 +7,7 @@ dependencies:
   - pip
   - pip:
     - jupyter-book==1.0.4post1
-    - quantecon-book-theme==0.9.2
+    - quantecon-book-theme==0.15.0
     - sphinx-tojupyter==0.3.1
     - sphinxext-rediraffe==0.2.7
     - sphinx-exercise==1.0.1