Add seeking example to explainer / samples

In response to user request: "How do we "seek" with the audio decoder?"

The recommendation is

1. call decoder.reset() to drop any pending/queued work
2. call decoder.configure(...) since reset() requires that. if the seek target has the same config as before, simply provide that config again to configure().
3. call decoder.decode(...) with packets starting at the timestamp you're seeking to. Note, you'll need to start with a key frame, but this is mostly not a worry for audio. For video, a typical approach (like used for <video>) would be to start with the key frame at-or-before your seek time, then discard outputs at the output callback until you see the first output with the time you desire.

Start by locating the explainer and samples referenced in the issue, then inspect how audio decoder examples are organized. Add a seeking example that demonstrates the reset, configure, and decode sequence described here, including the key-frame guidance, and verify that the sample makes the expected seeking behavior clear.