Add copying utility methods to Arena

src/connection.hpp

@@ -44,7 +44,7 @@ struct MessageSender {
    *
    * Example usage:
    * ```cpp
-   * auto response_parts = std::span{arena.allocate<std::string_view>(2), 2};
+   * auto response_parts = arena.allocate_span<std::string_view>(2);
    * response_parts[0] = "HTTP/1.1 200 OK\r\n\r\n";
    * response_parts[1] = "Hello World";
    * conn.append_message(response_parts, std::move(arena));
@@ -121,7 +121,7 @@ struct Connection : MessageSender {
    * Example usage:
    * ```cpp
    * Arena arena;
-   * auto* parts = arena.allocate<std::string_view>(2);
+   * auto parts = arena.allocate_span<std::string_view>(2);
    * parts[0] = build_header(arena);
    * parts[1] = build_body(arena);
    * conn.append_message({parts, 2}, std::move(arena));