build(deps): bump ansys-sphinx-theme in /requirements

Bumps [ansys-sphinx-theme](https://github.com/ansys/ansys-sphinx-theme) from 0.15.2 to 0.16.0.
- [Release notes](https://github.com/ansys/ansys-sphinx-theme/releases)
- [Commits](ansys/ansys-sphinx-theme@v0.15.2...v0.16.0)

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

Signed-off-by: dependabot[bot] <[email protected]>

requirements/requirements_doc.txt

@@ -11,7 +11,7 @@ vtk==9.3.0
 # Documentation dependencies
 Sphinx==7.3.7
 numpydoc==1.7.0
-ansys-sphinx-theme==0.15.2
+ansys-sphinx-theme==0.16.0
 sphinx-copybutton==0.5.2
 jupyter_sphinx==0.5.3
 jupyterlab>=3.2.8