Add documentation throughout the codebase.

[zleyyij]

I wanted to see how feasible it would be to implement a compressor for this crate, so I went through the code and added documentation for most things. Whether or not I implement a compressor in the future will depend on my motivation, but at the very least, I can contribute doc improvements.

[KillingSpark]

Very nice thanks! I'll give it a read soon.

About the compressor side of things: I would definitely accept a very basic compressor. I toyed with the idea of implementing an encoder that only supports RAW and RLE blocks. Even a "bad" compressor would enable interoperability between software that is expected to exchange data in zstd frames.

[KillingSpark]

Hi just a few things to fix. Sorry for not making it one review I probably spammed your inbox a bit :/

I want to thank you again, that's a lot of good doc!

[zleyyij]

All requested changes have been applied, there are a few things I think you should double check, but I've done everything you asked

[KillingSpark]

Looks good to me, the two CI errors should be easy fixes.

The doc comment should be fixable by not making it a file but just creating a reader with a todo!()

let reader: &mut Read = todo!("Get a reader from somewhere, like a file");

[KillingSpark]

Thanks!