Consider moving the tutorial off Read the Docs

[meooow25]

RTD broke recently (#1090), which makes me think that we should consider more manageable alternatives (#1090 (comment)).

• Plain markdown files in this repo
• The GitHub wiki feature (https://github.com/haskell/containers/wiki)
• A tutorial module (e.g. Pipes.Tutorial).

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