Improve Documentation on Writing Custom ASGI Middleware and BaseHTTPMiddleware

[erewok]

In light of the issues and PRs relating to BaseHTTPMiddleware we need to improve the documentation on this middleware class and give some good guidance on how to write custom middleware. Documentation on this should include the following:

• A warning not to use BaseHTTPMiddleware with StreamingResponse or FileResponse endpoints.
• A note on how background will be evaluated with respect to BaseHTTPMiddleware.
• Instructions on how to write custom ASGI Middleware with example code (can be based on an example from the codebase).
• A gentle suggestion to favor custom ASGI Middleware over subclassing BaseHTTPMiddleware.

See related issues and PRs:

• Background tasks don't work with middleware that subclasses BaseHTTPMiddleware #919
• Memory usage streaming large responses #1012
• "Task was destroyed but it is pending!" error when middleware dispatch func discards response from call_next #1022
• Remove maxsize arg from Queue constructor in BaseHTTPMiddleware #1028

[Kludex]

I think we should also document on how to create middlewares without BaseHTTPMiddleware.