Improve documentation for the fullstack crate (#2952)

* improve documentation for the fullstack server context

* Add a section about axum integration to the crate root docs

* make serve_dioxus_application accept the cfg builder directly

* remove unused server_fn module

* improve fullstack config docs

* improve documentation for the server function macro

* fix axum router extension link

* Fix doc tests

* Fix launch builder

* Remove redundant server_only inside server function

* Fix isrg config path

packages/fullstack/docs/request_origin.md

@@ -0,0 +1,40 @@
+This method interacts with information from the current request. The request may come from:
+
+1. The initial SSR render if this method called from a [`Component`](dioxus_lib::prelude::component) or a [`server`](crate::prelude::server) function that is called during the initial render
+
+```rust
+#[component]
+fn PrintHtmlRequestInfo() -> Element {
+    // The server context only exists on the server, so we need to put it behind a server_only! config
+    server_only! {
+        // Since we are calling this from a component, the server context that is returned will be from
+        // the html request for ssr rendering
+        println!("headers are {:?}", server_context().request_parts().headers);
+    }
+    rsx! {}
+}
+```
+
+2. A request to a [`server`](crate::prelude::server) function called directly from the client (either on desktop/mobile or on the web frontend after the initial render)
+
+```rust
+#[server]
+async fn read_headers() -> Result<(), ServerFnError> {
+    // Since we are calling this from a server function, the server context that is may be from the
+    // initial request or a request from the client
+    println!("headers are {:?}", server_context().request_parts().headers);
+    Ok(())
+}
+
+#[component]
+fn CallServerFunction() -> Element {
+    rsx! {
+        button {
+            // If you click the button, the server function will be called and the server context will be
+            // from the client request
+            onclick: move |_| read_headers().await,
+            "Call server function"
+        }
+    }
+}
+```