Docs: Provide examples of how to create middleware

[marcuslimdw]

Summary

For those used to thinking of a server as a function, which Starlette's BaseHTTPMiddleware does (caveat: it has its own problems), ASGI middleware can be quite confusing for two reasons:

1. The mapping of ASGI concepts to HTTP request/response cycle concepts, at a level that the average dev will be aware of, is not clear (what are send and receive, and how do they relate to requests and responses?)
2. Many operations are made much easier by not inspecting the raw scope dict, but rather performing framework-provided transformations (e.g. litestar.datastructures.Headers.from_scope or litestar.datastructures.URL.from_scope) on scope. These transformations are not inherently discoverable.

My suggestion: add multiple examples to the creating Middleware page, like Starlette already does.