Bump sphinx from 8.0.2 to 8.1.0 in /docs/readthedocs (#1167)

docs/readthedocs/requirements.txt

@@ -1,6 +1,6 @@
 commonmark==0.9.1
 recommonmark==0.7.1
-Sphinx==8.0.2
+Sphinx==8.1.0
 sphinx-rtd-theme==3.0.1
 myst-parser==4.0.0
 markdown-it-py==3.0.0