Bump ansys-sphinx-theme from 0.13.3 to 0.15.2 in /requirements (#1497)

Bumps [ansys-sphinx-theme](https://github.com/ansys/ansys-sphinx-theme) from 0.13.3 to 0.15.2.
- [Release notes](https://github.com/ansys/ansys-sphinx-theme/releases)
- [Commits](ansys/ansys-sphinx-theme@v0.13.3...v0.15.2)

---
updated-dependencies:
- dependency-name: ansys-sphinx-theme
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

requirements/requirements_docs.txt

@@ -1,4 +1,4 @@
-ansys_sphinx_theme==0.13.3
+ansys_sphinx_theme==0.15.2
 enum-tools[sphinx]==0.12.0
 graphviz==0.20.1
 imageio==2.28.0