Update documentation for 0.8

[minghuaw]

• [x] Add new book chapter on PubSub
• [x] Update the chapter on timeout
• [x] Add sections about #[export_trait(impl_for_client)]
• [x] Add description about anyhow feature flag
• [x] Add subchapter about integration with axum
• [x] Add changes of 0.8.0-alpha to the Change Log
• [x] Add changes of 0.8.0-beta to the Change Log
• [ ] Update book about feature flag "ws_tokio", "ws_async_std"
• [x] Update pubsub chapter on customizing Topic derive macro with the #[topic()] attribute