Bump ansys-sphinx-theme from 0.4.2 to 0.5.1 in /requirements

Bumps [ansys-sphinx-theme](https://github.com/ansys/ansys-sphinx-theme) from 0.4.2 to 0.5.1.
- [Release notes](https://github.com/ansys/ansys-sphinx-theme/releases)
- [Commits](ansys/ansys-sphinx-theme@v0.4.2...v0.5.1)

---
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

@@ -2,7 +2,7 @@ Sphinx==5.1.0
 jupyter_sphinx==0.4.0
 numpydoc==1.4.0
 matplotlib==3.5.1
-ansys-sphinx-theme==0.4.2
+ansys-sphinx-theme==0.5.1
 pypandoc==1.8.1
 pytest-sphinx==0.4.0
 sphinx-autobuild==2021.3.14