add rewritten router book and fix features code

docs/router/src/features/navigation/external.md

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+# External Navigation
+
+In modern apps, and especially on the web, we often want to send our users to an
+other website. [`External`] allows us to make a [`Link`] navigate to an
+external page.
+
+> You might already now about
+> [external navigation failures](../failures/external.md). The [`Link`]
+> component doesn't rely on the code path where those originate. Therefore a
+> [`Link`] will never trigger an external navigation failure.
+
+Strictly speaking, a [`Link`] is not necessary for navigating to external
+targets, since by definition the router cannot handle them internally. However,
+the [`Link`] component is more convenient to use, as it automatically sets the
+`rel` attribute for the link, when the target is external.
+
+## Code Example
+```rust
+# // Hidden lines (like this one) make the documentation tests work.
+# extern crate dioxus;
+use dioxus::prelude::*;
+# extern crate dioxus_router;
+use dioxus_router::prelude::*;
+# extern crate dioxus_ssr;
+
+fn App(cx: Scope) -> Element {
+    use_router(
+        &cx,
+        &|| RouterConfiguration {
+            # synchronous: true,
+            ..Default::default()
+        },
+        &|| Segment::empty()
+    );
+
+    cx.render(rsx! {
+        // links need to be inside a router, even if they navigate to an
+        // external page
+        Link {
+            target: NavigationTarget::External("https://dioxuslabs.com/".into()),
+            "Go to the dioxus home page"
+        }
+        Link {
+            target: "https://dioxuslabs.com/", // short form
+            "Go to the dioxus home page 2"
+        }
+    })
+}
+#
+# let mut vdom = VirtualDom::new(App);
+# vdom.rebuild();
+# let html = dioxus_ssr::render(&vdom);
+# assert_eq!(
+#     html,
+#     format!(
+#         "<a {attr1} {attr2}>{text}</a><a {attr1} {attr2}>{text} 2</a>",
+#         attr1 = r#"href="https://dioxuslabs.com/" dioxus-prevent-default="""#,
+#         attr2 = r#"class="" id="" rel="noopener noreferrer" target="""#,
+#         text = "Go to the dioxus home page"
+#     )
+# )
+```
+
+> Note that the short form for an [`ExternalTarget`] looks like the short form
+> for an [`InternalTarget`]. The router will create an [`ExternalTarget`] only
+> if the URL is absolute.
+
+[`External`]: https://docs.rs/dioxus-router-core/latest/dioxus_router_core/navigation/enum.NavigationTarget.html#variant.External
+[`Internal`]: https://docs.rs/dioxus-router-core/latest/dioxus_router_core/navigation/enum.NavigationTarget.html#variant.Internal
+[`Link`]: https://docs.rs/dioxus-router/latest/dioxus_router/components/fn.Link.html