Add Dataset.example_media and cache Datacards

[hagenw]

Add caching for results (e.g. PNG files) generated by audbcards.Datacard.

To achieve this, I did the following:

• Move audbcards.Datacard.example_media to audbcards.Dataset.example_media as it requires access to the dependency table (audbcards.Dataset.deps), and hence it is better suited to be cached as part of audbcards.Dataset.
• Added cache_root argument to audbcards.Datacard. It uses per default the same folder as audbcards.Dataset.
• Cache the resulting plot of audbcards.Datacard.file_duration_distribution() in the cache folder.
• Cache the media file and resulting plot from audbcards.Datacard.player() in the cache folder.

The structure of the stored cache files is (shown by the example for emodb):

$ tree ~/.cache/audbcards/emodb
.../.cache/audbcards/emodb
└── 1.4.1
    ├── emodb-1.4.1-file-duration-distribution.png
    ├── emodb-1.4.1-player-media
    │   └── wav
    │       └── 13b09La.wav
    ├── emodb-1.4.1-player-waveform.png
    └── emodb-1.4.1.pkl

I again tested building the pages for all our datasets and now get:

branch	fresh build	build from cache
main	15 minutes	2 minutes
this branch	15 minutes	30 seconds

whereas now most of the time is spend on compiling the HTML pages, and not on gathering information about the datasets.

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Updated docstrings:

[hagenw]

The error you see with Python 3.12 is related to caused by https://github.com/devopshq/artifactory/issues/430. Until that is solved all our packages using Artifactory backends are blocked for Python 3.12.

[hagenw]

For audbackend 2.0.0, I prepared already https://github.com/audeering/audbcards/pull/90, but I wanted to first get this here merged, so I can rebase my changes for audbbackend 2.0.0, before assigning it for review.

[ChristianGeng]

For me everything is addressed. I do not know how to later approve in github. I would if I did though. Anyway I am happy now and this can imo be merged.

[hagenw]

Thanks. I also don't know how to later approve. I guess it is implicitly assumed that it is approved when all discussions are resolved. Otherwise, I would have to re-request a review, and then you could approve.