Enable intersphinx extension

[1ucian0]

From the conversation in https://github.com/Qiskit/qiskit-terra/pull/9771/files#r1132366803:

I believe that the :class: directive won't really create a link for matplotlib.figure, PIL.Image and str, because these are not classes documented in the scope ofqiskit-terra. If I'm not misteken, It will just have the same effect as writing the text between backticks. Therefore... wouldn't it be clearer to use the backticks directly? (also, shouldn't matplotlib.figure be matplotlib.figure.Figure ?)

            :class:`.TextDrawing` or ``matplotlib.figure.Figure`` or ``PIL.Image`` or
            ``str``:

FYI, you can add links to external documentation by:

1. If they use sphinx to build their docs, you can use intersphinx_mapping
2. Manually adding a link to whatever URLs are of interest.

Originally posted by @ElePT in Qiskit/qiskit#9771 (comment)

[jakelishman]

Fixed by #1769.