Update README and lib.rs example to use WebDriver::managed

The crate's docs in the book and examples already lead with
WebDriver::managed; the top-level README and crate-root rustdoc were
still showing the chromedriver-on-a-port flow. Switch them over so the
first thing a new reader sees matches the recommended path.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: stevepryde

thirtyfour/README.md

@@ -44,7 +44,7 @@ use thirtyfour::prelude::*;
 #[tokio::main]
 async fn main() -> WebDriverResult<()> {
      let caps = DesiredCapabilities::chrome();
-     let driver = WebDriver::new("http://localhost:9515", caps).await?;
+     let driver = WebDriver::managed(caps).await?;
 
      // Navigate to https://wikipedia.org.
      driver.goto("https://wikipedia.org").await?;

thirtyfour/src/lib.rs

@@ -36,15 +36,17 @@
 //! ## Example
 //!
 //! The following example assumes you have a compatible version of Chrome
-//! installed and `chromedriver` running on port 4444.
+//! installed. [`WebDriver::managed`] auto-downloads the matching
+//! `chromedriver`, starts it locally, and shuts it down when the session is
+//! dropped.
 //!
 //! ```no_run
 //! use thirtyfour::prelude::*;
 //!
 //! #[tokio::main]
 //! async fn main() -> color_eyre::Result<()> {
 //!     let caps = DesiredCapabilities::chrome();
-//!     let driver = WebDriver::new("http://localhost:4444", caps).await?;
+//!     let driver = WebDriver::managed(caps).await?;
 //!
 //!     // Navigate to https://wikipedia.org.
 //!     driver.goto("https://wikipedia.org").await?;