Enhance Documentation: Alternatives to select! Macro

[Ddystopia]

This PR adds documentation for ecosystem alternatives to the select! macro, such as tokio_stream::StreamExt::merge and futures-concurrency utilities. Providing these alternatives helps users make informed decisions by showcasing additional tools and patterns, as a lot of new users are not aware of any alternatives to select!.

[Ddystopia]

Are there some issues with pr?

[Darksonn]

Sorry, I forgot to click submit on my review.

My most general comment is that I'm not sure the selection of alternatives is complete. Surely we cannot discuss alternatives to the tokio::select! macro without mentioning the futures crate? It also has a macro called select! that is different from our macro in various ways.

More generally, Tokio usually links to futures over futures-lite because the futures crate is an official rust-lang project. As for futures-concurrency, I am okay with linking to that since the direct purpose of that crate is to investigate alternatives to select, so it is obvious to mention it.

[Ddystopia]

Thank you for the review, I've changed examples, moved the whole section after select! examples and focused motivation on cancellation safety issues

[Ddystopia]

Oh, there is an infinite loop in example 😅