documented base async middleware

Author: amirreza8002

docs/middleware/base.md

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+## AsyncMiddlewareMixin
+
+a base class to create async only middlewares.
+
+it is very similar to django's [MiddlewareMixin](https://docs.djangoproject.com/en/5.1/topics/http/middleware/#upgrading-pre-django-1-10-style-middleware)
+with the following specification:
+
+`__call__` method is async, so you need to `await` the middleware instance.
+```pycon
+>>> middleware = AsyncMiddlewareMixin(get_response)
+>>> await middleware()
+```
+where `get_response` is an async function, sync functions are not supported and will raise an error.
+
+----------------------------
+
+other methods are as follows:
+
+* `process_request` and `process_response` are `await`ed inside the middleware and have to be async
+
+* `process_view` and `process_template_response` can be either sync or async, but **async is preferred**, if it's sync django will wrap it as async which might have slight performance reduction.
+
+* `process_exception` can be either sync or async, but **sync is preferred**, if async is used django wraps the method to be called synchronously.