Execute examples in parallel - #900

This PR makes the doc building execute the examples in parallel, significantly speeding up the overall execution.

This has been achieved as follows:

• There is now a dedicated function for executing and converting a single example (_convert_example)
• We do some Thread-magic that Claude came up with 😆
• The whole job now fails when a single example fails, and it fails faster with a clearer error message, see here where I added an error into the basic example: https://github.com/AVHopp/baybe_dev/actions/runs/33525219751/job/99914223793 This does make the whole output a bit more verbose, but I think it is worth it (in particular since this makes it easier when just routing the action to an agent who can easily filter this)

Important: While this always improves the running time, it is hard to estimate by how much since this seems to depend on how much work load is currently done in other CI tests. I've seen the full doc building job to complete between 8 and 14 minutes, which is a speed-up in any case.

Compiled docs: See https://avhopp.github.io/baybe_dev/latest/. PLEASE TAKE A LOOK HERE at the examples - I looked at some of them and everything looked as expected, but please double-check that your favourite examples look like you expect them to look.

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