[Secrets] Add a section to give the best pratices for accessing secrets in a service

[tcoch]

Last week I was wondering what was the best way of handling this subject, and couldn't find the answer (though I found multiple ways of doing it with different technical approches).
I propose this PR to document this way of accessing secrets values inside a service, as the 'best pratice' if this is in fact agreed upon.

[lacatoire]

Hi @tcoch, thanks for your PR.
Since secrets are just regular environment variables, this is already covered by the autowiring docs. A full section would be redundant, but a seealso would help discoverability. Could you replace your changes with this instead?

  .. seealso::

      Since secrets are exposed as environment variables, you can inject them
      into services using the ``#[Autowire]`` attribute
      (e.g. ``#[Autowire(env: 'MY_SECRET')]``). See :doc:`/service_container/autowiring`
      for details.

Right after the .env.test code block (line 220), before the "Deploy Secrets to Production" section.