FEAT: linkable math from code-blocks

[mmcky]

a user would like to ability to for example have the following math and then use the label in code blocks for link-able references

.. math::
    :label: ma_inf

    X_t = \sum_{j=0}^{\infty} \psi_j \epsilon_{t-j},
    \qquad t \in \mathbb Z

such as

.. code-block: python
    
   <code> # :eq:`ma_inf`

we do have access to the contents of code-blocks but they do get moved out to pygments for rendering. So perhaps we can intercept on return of that process (in html format to insert the equation clickable link). I'll open as a FEATURE request on the sphinxcontrib-jupyter repo.

rather than looking for many tags it would be best to use a convention such as:

:rst:eq:`ma_inf`

[arnavs]

@mmcky Just wondering, has this been implemented yet/is it something that you think we could push forward on? It would be a big help as we're going through and adding math references to the code on the Julia side.

[mmcky]

@arnavs I don't see this being implemented until v0.6 release. Let me know if that is a problem.