Document the multi-stream demuxer in the blocks tutorial

[NicolasHug]

Two new sections in the example: metadata, showing the three tiers side by
side so the difference between a header value and a scanned one is visible
rather than described, and video-and-audio-together, showing a Demuxer
getting both out of a single pass.

The benchmark builds its decoder with demuxer.make_decoder(), which is the
documented path now. The module docstring drops the TODO asking how to expose
header metadata, since that is what the previous commit answered.

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[pytorch-bot]

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