Sphinx changed something between sphinx=4.5.0 and sphinx=5.3 that broke
"sphinxcontrib.log_cabinet" plugin.

When Sphinx tries to importlib.import_module("sphinxcontrib.log_cabinet"), it
has couple other sphinxcontrib.* libraries in sys.path, and they seem to cause
conflict. So here I purge sys.path from sphixcontrib-related things, import
log_cabinet and put everything back, so when Sphinx will do "import_module",
"sphinxcontrib.log_cabinet" will be already imported and cached.

All this is quite hacky, but we are talking about merely building documentation
here. If resulting html looks good, what happened in Nix sandbox stays in Nix
sandbox.

--- a/docs/conf.py	1970-01-01 00:00:00.000000000 -0000
+++ b/docs/conf.py	1970-01-01 00:00:00.000000000 -0000
@@ -1,5 +1,12 @@
 from pallets_sphinx_themes import get_version
 from pallets_sphinx_themes import ProjectLink
+import importlib
+import sys
+
+saved_path = sys.path
+sys.path = [x for x in sys.path if "sphinxcontrib" not in x or "cabinet" in x]
+import sphinxcontrib.log_cabinet
+sys.path = saved_path
 
 # Project --------------------------------------------------------------