Docs: Fix module docs for songbird::tracks.

Module docs mistakenly used the old doc-link format, so removing `create_player` never fired an error! These have also been partially rewritten to explain the role of `Track` and `TrackHandle`.

Includes some other misc fixes to links, mention of `TrackHandle::action` for metadata handling, etc.

Closes #140.

src/input/mod.rs

@@ -144,6 +144,8 @@ use tokio::runtime::Handle as TokioHandle;
 ///
 /// // If we want to inspect metadata (and we can't use AuxMetadata for any reason), we have
 /// // to parse the track ourselves.
+/// //
+/// // We can access it on a live track using `TrackHandle::action()`.
 /// in_memory_input = in_memory_input
 ///     .make_playable_async(&CODEC_REGISTRY, &PROBE)
 ///     .await