Consider moving the tutorial off Read the Docs

RTD broke recently (#1090), which makes me think that we should consider more manageable alternatives (https://github.com/haskell/containers/issues/1090#issuecomment-2587281333).

> * Plain markdown files in this repo
> * The GitHub wiki feature (https://github.com/haskell/containers/wiki)
> * A tutorial module (e.g. [Pipes.Tutorial](https://hackage.haskell.org/package/pipes-4.3.16/docs/Pipes-Tutorial.html)).

Opinions welcome.

Personally I like the idea of a Tutorial module. It's the option with the best visibility and links to functions and types work without any extra effort.

Related: This ongoing discussion on Discourse https://discourse.haskell.org/t/library-companion-site-tools/11276