Mention other ways to handle private key files (#57238)

Often when I am helping others learn Ansible, they are using nodes spun up on Amazon/Google/etc.  where private key files (pem) are needed.  Mentioning just a bit more on how to handle those private key files on the intro_getting_started page will help more folks get started faster.
I even reviewed the changes with my learning team and they all agreed this helped clarify immediately for them how to really get started with Ansible from a practical perspective.

docs/docsite/rst/user_guide/intro_getting_started.rst

@@ -64,8 +64,13 @@ do:
 
     $ ssh-agent bash
     $ ssh-add ~/.ssh/id_rsa
+    
+Depending on your setup, you may wish to use Ansible's ``--private-key`` command line option to specify a pem file instead.  You can also add the private key file:
 
-(Depending on your setup, you may wish to use Ansible's ``--private-key`` option to specify a pem file instead)
+    $ ssh-agent bash
+    $ ssh-add ~/.ssh/keypair.pem
+
+Another way to add private key files without using ssh-agent is using ``ansible_ssh_private_key_file`` in an inventory file as explained here:  :ref:`intro_inventory`.
 
 Now ping all your nodes: