C++ docs: Fix red warning in generated HTML for file and namespace listings

The Furo theme by default generates a "on this page" contents list in
the right column. The breathe output for C++ namespace and file pages
also includes a ::contents:: RST directive, to summarize the available
headings. The duplication of these two is warned about by furo with a
big fat red warning.

Solve this by not making breathe generate the contents directive
anymore. According to

    https://exhale.readthedocs.io/en/latest/reference/configs.html?highlight=contentsspecifier#exhale.configs.kindsWithContentsDirectives

it's by default used for files and namespace, so make it empty in our
config.

Fixes #2505

api/cpp/docs/conf.py

@@ -58,6 +58,7 @@ exhale_args = {
     ),
     "doxygenStripFromPath": "..",
     "createTreeView": True,
+    "kindsWithContentsDirectives": [],
     "exhaleExecutesDoxygen": True,
     "exhaleDoxygenStdin": """INPUT = ../../api/cpp/include generated_include
 EXCLUDE_SYMBOLS = slint::cbindgen_private* slint::private_api* vtable* slint::testing* SLINT_DECL_ITEM