Fix the CI failures of the MessagePack length check

- mark to_msgpack_length's value as used when exceptions are disabled
  (-Wunused-parameter, misc-unused-parameters)
- put "Exception safety" before "Exceptions" in to_msgpack.md, as the
  documentation style check requires
- create the test's string value from its type: constructing it from a
  beyond_uint32_string_t considers the std::filesystem::path conversion,
  which libstdc++ 10 reports as ambiguous for a class derived from
  std::string (clang 13)

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

single_include/nlohmann/json.hpp

@@ -19944,6 +19944,7 @@ class binary_writer
             JSON_THROW(out_of_range::create(412, concat("MessagePack length ", std::to_string(length), " exceeds maximum of ", std::to_string((std::numeric_limits<std::uint32_t>::max)())), &j));
         }
 
+        static_cast<void>(j);
         return static_cast<std::uint32_t>(length);
     }