chore(deps): fix docs deps

These should have been updated by postgrest-nixpkgs-upgrade, when I
upgraded the Nixpkgs pin, but I did not follow my own readme, and just
ran nix flake update...

Author: wolfgangwalther

docs/requirements.txt

@@ -1,7 +1,7 @@
 # This file is auto-generated by postgrest-nixpkgs-upgrade
-sphinx==8.2.3
+sphinx==9.1.0
 sphinx-copybutton==0.5.2
 sphinx-rtd-dark-mode==1.3.0
-sphinx-rtd-theme==3.0.2
-sphinx-tabs==3.4.7
-sphinxext-opengraph==0.9.1
+sphinx-rtd-theme==3.1.0
+sphinx-tabs==3.5.0
+sphinxext-opengraph==0.13.0