[Fleet] Add docs for package verification GPG key (#144707)

* Add docs for package verification GPG key

* Update fleet-settings.asciidoc

* Update docs/settings/fleet-settings.asciidoc

Co-authored-by: DeDe Morton <dede.morton@elastic.co>

Co-authored-by: DeDe Morton <dede.morton@elastic.co>

docs/settings/fleet-settings.asciidoc

@@ -33,6 +33,9 @@ The address to use to reach the {package-manager} registry.
 The proxy address to use to reach the {package-manager} registry if an internet connection is not directly available.
 Refer to {fleet-guide}/air-gapped.html[Air-gapped environments] for details.
 
+`xpack.fleet.packageVerification.gpgKeyPath`::
+The path on disk to the GPG key used to verify {package-manager} packages. If the Elastic public key
+is ever reissued as a security precaution, you can use this setting to specify the new key.
 
 ==== {fleet} settings