AddMember requires allocator, this fix keeps all uses of AddMember consistent across the whole tutorial.

Reorganized some features. Try to make the text clearer in features and
tutorial.

In the original disambiguation fix for `GenericValue::operator[]` (#170),
the documentation has been missing, which led to quite badly rendered
Doxygen pages.

During a cleanup, I've realized that a much simpler disambiguation is
possible:

````cpp
  GenericValue& operator[](SizeType idx); // array
  template <typename T>
  GenericValue& operator[](T* name);      // object
````

This approach works, as non-template functions are preferred over
template functions.

In order to improve the error messages, the pointer type is restricted
to `(const) Ch`.

Update `tutorial.md` to drop the ambiguity warning.

Correct `PushBack()` example

Some links will become broken in GitHub.

Imitate readthedocs

Fixes table