build(deps): bump ansys-sphinx-theme in /requirements (#631)

Bumps [ansys-sphinx-theme](https://github.com/ansys/ansys-sphinx-theme) from 1.3.2 to 1.6.3.
- [Release notes](https://github.com/ansys/ansys-sphinx-theme/releases)
- [Commits](ansys/ansys-sphinx-theme@v1.3.2...v1.6.3)

---
updated-dependencies:
- dependency-name: ansys-sphinx-theme
  dependency-version: 1.6.3
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Roberto Pastor Muela <37798125+RobPasMue@users.noreply.github.com>

Author: dependabot[bot]

requirements/requirements_doc.txt

@@ -1,5 +1,5 @@
-Sphinx<10
-ansys-sphinx-theme==1.3.2
+Sphinx<9
+ansys-sphinx-theme==1.6.3
 imageio==2.37.0
 nest-asyncio==1.6.0
 notfound==1.0.2