Rework Guide (#486)

* Move getting started to Guide

* Clean up summary and getting started

* Shorten intro by moving stuff to individual platform docs

* Make intro even shorter, summarize features

* Further cleanup; move development-related docs to separate section

* Make `guide` a crate. Turn code samples into examples so that we can check if they compile

* Rewrite "Describing the UI":

* Focus on RSX syntax (interactivity covered in later chapters); make sure samples are tested; concise language

* Move some "special attribute" samples to the `examples` directory

* Simplify introduction to components

* Simplify introduction to component props

* Document Prop features; add code samples

* Simplify component children docs

* Interactivity: better introduction to events

* Hooks: better introduction

* Remove outdated doc

* Introducs use_ref

* Simplify User Input chapter

* Document event handler props

* Meme editor example

* Meme editor walkthrough

* Add dark mode example

* Guide for context; dark mode example

* Guide: custom hooks

* Guide: conditional rendering

* Guide: rendering lists

* Guide: rendering lists + keys

* Move remaining infor from Reference to guide

* Delete reference book

docs/guide/examples/spawn.rs

@@ -0,0 +1,84 @@
+#![allow(non_snake_case, unused)]
+
+use dioxus::prelude::*;
+
+fn main() {
+    dioxus::desktop::launch(App);
+}
+
+fn App(cx: Scope) -> Element {
+    // ANCHOR: spawn
+    let logged_in = use_state(&cx, || false);
+
+    let log_in = move |_| {
+        cx.spawn({
+            let logged_in = logged_in.to_owned();
+
+            async move {
+                let resp = reqwest::Client::new()
+                    .post("http://example.com/login")
+                    .send()
+                    .await;
+
+                match resp {
+                    Ok(_data) => {
+                        println!("Login successful!");
+                        logged_in.set(true);
+                    }
+                    Err(_err) => {
+                        println!(
+                            "Login failed - you need a login server running on localhost:8080."
+                        )
+                    }
+                }
+            }
+        });
+    };
+
+    cx.render(rsx! {
+        button {
+            onclick: log_in,
+            "Login",
+        }
+    })
+    // ANCHOR_END: spawn
+}
+
+pub fn Tokio(cx: Scope) -> Element {
+    let _ = || {
+        // ANCHOR: tokio
+        cx.spawn(async {
+            let _ = tokio::spawn(async {}).await;
+
+            let _ = tokio::task::spawn_local(async {
+                // some !Send work
+            })
+            .await;
+        });
+        // ANCHOR_END: tokio
+    };
+
+    None
+}
+
+pub fn ToOwnedMacro(cx: Scope) -> Element {
+    let count = use_state(&cx, || 0);
+    let age = use_state(&cx, || 0);
+    let name = use_state(&cx, || 0);
+    let description = use_state(&cx, || 0);
+
+    let _ = || {
+        // ANCHOR: to_owned_macro
+        use dioxus::core::to_owned;
+
+        cx.spawn({
+            to_owned![count, age, name, description];
+            async move {
+                // ...
+            }
+        });
+        // ANCHOR_END: to_owned_macro
+    };
+
+    None
+}