Bump ansys-sphinx-theme from 0.5.2 to 0.6.1 in /requirements

Bumps [ansys-sphinx-theme](https://github.com/ansys/ansys-sphinx-theme) from 0.5.2 to 0.6.1.
- [Release notes](https://github.com/ansys/ansys-sphinx-theme/releases)
- [Commits](ansys/ansys-sphinx-theme@v0.5.2...v0.6.1)

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

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

requirements/requirements_docs.txt

@@ -1,7 +1,7 @@
 pypandoc==1.9
 sphinx==5.1.1
 pyvista==0.36.1
-ansys_sphinx_theme==0.5.2
+ansys_sphinx_theme==0.6.1
 imageio-ffmpeg==0.4.7
 nbsphinx==0.8.9
 pytest-sphinx==0.5.0