source links in stable API reference link to code on main branch

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The Python API reference has a handy link for finding the source code associated with each documented class/function. However, after splitting our docs into stable (latest release) and dev (current main branch), this functionality is no longer accurate for the stable docs. The link always points to the main branch, instead of the code for the latest release.

The code for this functionality lives here and was introduced by me in this PR.

Fixing this is not trivial. We must somehow determine whether we're building docs for stable or for dev. I believe pandas does this through their __version__ specifier. We should find a way that works for us.

This issue was pointed out to me by @corneliusroemer.

[stinodego]

This was resolved in #10542