Suppress epub warnings for duplicated ToC entries.

These fail RTD builds like https://readthedocs.org/projects/python-jsonschema/builds/18708083/
Not sure what's new (and don't care much to investigate at the minute).

But it seems this is somewhat expected behavior, see:

sphinx-doc/sphinx#4723 (comment)

docs/conf.py

@@ -165,6 +165,7 @@ def entire_domain(host):
 suppress_warnings = [
     "autoapi.python_import_resolution",
     "autoapi.toc_reference",
+    "epub.duplicated_toc_entry",
 ]
 autoapi_root = "api"
 autoapi_ignore = [