Provide documentation in the middleware module

[Nerixyz]

PR Type

Other/Documentation

PR Checklist

• Tests for the changes have been added / updated.
• Documentation comments have been added / updated.
• A changelog entry has been made for the appropriate packages.
• Format code with the latest stable rustfmt.
• (Team) Label with affected crates and semver status.

Overview

As explained in #3026, this provides documentation about middleware in the middleware module. I took the example and parts of the explanation from actix.rs/docs/middleware. I added a section on the traits and the order of middleware.

Furthermore, I looked at the authors-guide.md and tried to include some sections from there. I'm still missing "Understanding Body Types", "Best Practices", "Error Propagation", and "When To (Not) Use Middleware". Should I include these in this PR?

Since the middleware section now contains an example, should the documentation for Transform link to it and/or should the example there be removed/changed?

Closes #3026

[robjtede]

I cannot thank you enough for getting this kicked off. I've been dreading writing this out but this is a fantastic basis and deserves to be merged ASAP.

I've gone over it, making mostly mechanical changes. The exception is that I've replaced the section on wrap_fn with some notes about the from_fn helper from actix-web-lab as it's more the direction we want to point newcomers, more capable, and much easier to use.

This gives me motivation to flesh out the remaining sections in authors-guide.md file using this as a base. Thanks again. (Seriously, beer on me if you come to EuroRust 😛)