Removed usage of functions deprecated since 0.7

[Nevalicjus]

As per the docs, some functions exposed by leptos::prelude are deprecated to conform to Rust idioms. I've removed their usage in docs, examples, tests, etc., as per the deprecated notes:

• create_effect -> Effect::new
• create_memo -> Memo::new
• create_rw_signal -> RwSignal::new
• create_signal -> signal
• create_Action -> Action::new
• create_node_ref -> NodeRef::new
• watch -> Effect::watch

[gbj]

Thanks very much!