Bump Sphinx, use tagged release of Furo

Author: kberzinch

docs/poetry.lock

@@ -8,12 +8,12 @@ package-mode = false
 
 [tool.poetry.dependencies]
 python = "^3.12"
-sphinx = "8.2.3"
+sphinx = "9.1.0"
 sphinx-autobuild = "2025.8.25"
 sphinxext-opengraph = "0.13.0"
 sphinxcontrib-httpdomain = "1.8.1"
 proselint = "0.16.0"
-furo = {git = "https://github.com/pradyunsg/furo"}
+furo = "2025.12.19"
 
 [build-system]
 requires = ["poetry-core"]