nix: keep readthedocs dependencies in-sync with nix

This is to make sure that we will always have the same development
environment for the docs build as is used live on the website.

docs/requirements.txt

@@ -1,9 +1,7 @@
-docutils==0.20.1
-sphinx-copybutton
-sphinx-intl
-sphinx-rtd-theme>=0.5.1
-sphinx-rtd-dark-mode>=1.3.0
-sphinx-tabs>=3.2.0
-sphinx>=5.0.2
-sphinxext-opengraph==0.9.1
-urllib3==2.2.3
+# This file is auto-generated by postgrest-nixpkgs-upgrade
+sphinx==7.4.7
+sphinx-copybutton==0.5.2
+sphinx-rtd-dark-mode==1.3.0
+sphinx-rtd-theme==2.0.0
+sphinx-tabs==3.4.7
+sphinxext-opengraph==0.9.1