doc: improve async_context introduction

- Rename "Async_context" to "Asynchronous context tracking" in toc.
- Use named imports to show how `AsyncLocalStorage` and `AsyncResource`
  can be imported.

PR-URL: https://github.com/nodejs/node/pull/40560


Reviewed-By: Vladimir de Turckheim <vlad2t@hotmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Voltrex <mohammadkeyvanzade94@gmail.com>
Reviewed-By: Zijian Liu <lxxyxzj@gmail.com>
Reviewed-By: Gerhard Stöbich <deb2001-github@yahoo.de>