move BART case studies into seperate category

[daniel-saunders-phil]

The case studies section of the examples should be subdivided into more specific categories. BART is a distinct type of modeling rather than a case study. We have enough notebooks for it now that it can stand on its own.

• update thumbnail_extractor
• standardize titles BART vs bart

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📚 Documentation preview 📚: https://pymc-examples--597.org.readthedocs.build/en/597/

[juanitorduz]

@aloctavodia does this have any effect on BART docs (because of the paths?)

[OriolAbril]

we can set up redirects to preserve the old paths, but bart docs should use the (target)= at the top of the notebook to reference them for things like that; if the target doesn't change, the cross-reference is still valid.

Links from discourse to notebooks and stuff like that can't use cross-references, so redirects are a good idea

[OriolAbril]

add redirects here: https://github.com/pymc-devs/pymc-examples/blob/main/examples/conf.py#L155; keys are old/unexistent paths (redirect origin), values are existing paths (redirect endpoint)

[twiecki]

@OriolAbril I don't think we need to add redirects, if people want stable links they should reference a specific version of the docs.

[aloctavodia]

Thanks, @daniel-saunders-phil for the PR. I am working on BART right now I will update the paths if necessary.