<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def rst_filename_rel_autodoc_index(self, index_filename: str) -> str: """ Returns the filename of the target RST file, relative to a specified index file. Used to make the index refer to the RST. """

        index_dir = dirname(abspath(expanduser(index_filename)))
        return relpath(self.target_rst_filename, start=index_dir)