feat: add core types and update documentation

[polw1]

This PR adds documentation for the media-parser crate, including examples of how to use the public API.
It also adds the Rust code definitions for the core types and their structs.

Key features covered in the documentation:

• Retrieving metadata (title, artist, album, duration, etc.)
• Listing tracks with type, codec, and language
• Extracting subtitles by track ID or language
• Capturing single or multiple video frames

[velocitysystems]

read and seek should probably be modified to not take &mut self as this complicates ownership.

We may be able to use interior mutability Arc<Mutex<>> to hide the mutable state from the public API.

[polw1]

I wonder if pushing mutability inside the reader could hurt performance.

For example:

reader.seek(SeekFrom::Start(100)).await?;
reader.read(&mut buf).await?; // lock during async I/O

To avoid a race between seek and read, we’d need to hold the same lock across both operations—which means holding a lock while awaiting I/O.

Considering how we possibly will use the reader, i think it makes sense to use a read_at/size approach and avoid all these problems. What do you think?

[velocitysystems]

Good point, @polw1. Agree with your suggestion to update the StreamReader trait with a read_at function to eliminate the need for interior mutability. This not only resolves the mutability issue but also enables parallel I/O reads without locks.

It also aligns with patterns used in other crates to support stateless reads with parallel access.

[velocitysystems]

Handing over to @yokuze for review.