Fixes SSH public key auth, misc document clean-up

[msheiny]

What this does

This PR

• fixes code logic to make ssh public key authorization possible.

• makes small tweak to the document

• Fixes SSH keypair auth does not work with the SSOTSyncDevices job #293

• Addresses one-bullet point of Onboarding docs - out-of-date references, assumptions #291 - not 100% sure the best place to address the rest. Kinda seems like some of it is a core nautobot fix to expose filters to the user that are being applied when searching for a role/location/etc.

Testing

1. Made the following changes to my nautobot config:

+++ b/development/nautobot_config.py
@@ -152,6 +152,8 @@ PLUGINS_CONFIG = {
                 "extras": {
                     "fast_cli": False,
                     "read_timeout_override": 30,
+                    "use_keys": True,
+                    "key_file": "/source/.ssh_test/id_rsa",
                 },
             },
         },

2. Created an ssh rsa key at .ssh_test/id_rsa in this repo
3. Spun-up a containerlabs instance of an arista ceos and enabled SSH public key auth using that public key
4. Created a secretsgroup where i just specified the username (i also tried setting the password to a random value as suggested in the docs, but that seemed kinda pointless so I modified the docs accordingly)
5. Ran a job against the arista device and confirmed that login was successful

[msheiny]

dropped secret as it was not being utilized in any of the downstream code in this package. and i didnt see any signs that this specific function was being referenced in any other nautobot code