Document the Middleware type alias

[Profpatsch]

I tried to come at this from the angle of

1. A user of Wai trying to understand what Middleware is
   and how to use it, especially what happens if you chain
   multiple middlewares and in what order their are called.

2. A developer trying to implement new middleware, how to
   modify requests and responses, and how to pass on data.

by giving examples of how the type signatures and
simple usage/implementations look.

I think Wai is a super fundamental module for the Haskell
ecosystem, so we should aim for the documentation to be top
notch (think: rust stdlib).

I think the rest of the module needs a simple overhaul,
but I was especially annoyed by having to reconstruct Middleware
from first principles every time I built a server (“which order
does it chain again?”).

Before submitting your PR, check that you've:

• Bumped the version number
• Documented new APIs with Haddock markup
• Added @since declarations to the Haddock

After submitting your PR:

• Update the Changelog.md file with a link to your PR
• Check that CI passes (or if it fails, for reasons unrelated to your change, like CI timeouts)

[Profpatsch]

Please check especially the code examples, I tried most of them in a slightly modified form on the REPL, but I might have typoed something. The examples are not copy-pastable, which would be the gold standard, but I think pseudocode like this already helps a lot in practice.

[Profpatsch]

Rendered:

[Profpatsch]

Looks to me like the Windows CI failure is unrelated.

[snoyberg]

Overall looks good, some minor points added

[snoyberg]

Thanks!