message: memo: mentioned value function on main doc page
diff: @@ -27,6 +27,7 @@ These pages contain the API documentation of JSON for Modern C++, a C++11 header
  - @link nlohmann::basic_json::get_ptr get_ptr @endlink -- get a value pointer
  - @link nlohmann::basic_json::get_ref get_ref @endlink -- get a value reference
  - @link nlohmann::basic_json::operator ValueType() const operator ValueType @endlink -- get a value (implicit conversion)
  + - @link nlohmann::basic_json::value value @endlink -- get a value from an object and return default value if key is not present
  - lexicographical comparison operators
  - serialization
  - deserialization
  