Add pre-commit check for sphinx cross-references

[OriolAbril]

Now that we have versioned docs, we can't use urls for internal links if we want each version to point to themselves. I think we should add a pre-commit check to ensure there are no links pointing to docs.pymc.io anywhere inside the docs/source/ folder.

I don't know pre-commit but it seems like this should be easy for someone who does know their way around it. cc @MarcoGorelli if you agree with this, can you label it beginner friendly?

[MarcoGorelli]

yeah I think just a pygrep check for this would be good

[symeneses]

Hi, I would like to work on this issue ✋🏽

[OriolAbril]

Assigned it to you