Add middleware docs

[ElijahAhianyo]

The current documentation doesn't explain how middlewares work in cot(it only mentions it). It would be very helpful to have a dedicated page covering:

• An overview of what middlewares are and how they function within cot
• A list and explanation of the built-in middlewares provided out-of-the-box

Personal opinion:
Perhaps there could be an advanced section that talks about the middleware internals. This could cover topics such as how middlewares integrate into the request/response lifecycle and interact with the rest of the framework and maybe a walkthrough of how to create a custom one which would be valuable for downstream third-party library maintainers or anyone looking to understand the inner workings of cot more deeply.