[8.8] [Docs] Add new Fleet preconfiguration settings (#158771)

docs/settings/fleet-settings.asciidoc

@@ -52,12 +52,11 @@ Hostnames used by {agent} for accessing {es}.
 `xpack.fleet.agents.elasticsearch.ca_sha256`::
 Hash pin used for certificate verification. The pin is a base64-encoded string of the SHA-256 fingerprint.
 
-
 [role="child_attributes"]
 ==== Preconfiguration settings (for advanced use cases)
 
-Use these settings to pre-define integrations and agent policies that you
-want {fleet} to load up by default.
+Use these settings to pre-define integrations, agent policies, and {fleet-server}
+hosts or proxies that you want {fleet} to load up by default.
 
 NOTE: These settings are not supported to pre-configure the Endpoint and Cloud
 Security integration.
@@ -134,6 +133,41 @@ List of agent policies that are configured when the {fleet} app starts.
     Array that overrides any default input settings for this integration. Follows the same schema as integration inputs, with the exception that any object in `vars` can be passed `frozen: true` in order to prevent that specific `var` from being edited by the user.
 =======
 =====
++
+Example configuration:
++
+[source,yaml]
+----
+xpack.fleet.packages:
+  - name: apache
+    version: 0.5.0
+
+xpack.fleet.agentPolicies:
+  - name: Preconfigured Policy
+    id: 1
+    namespace: test
+    package_policies:
+      - package:
+          name: system
+        name: System Integration
+        id: preconfigured-system
+        inputs:
+          - type: system/metrics
+            enabled: true
+            vars:
+              - name: system.hostfs
+                value: home/test
+            streams:
+              - data_stream:
+                  dataset: system.core
+                enabled: true
+                vars:
+                  - name: period
+                    value: 20s
+          - type: winlog
+            enabled: false
+----
+
 
 `xpack.fleet.outputs`::
 List of outputs that are configured when the {fleet} app starts.
@@ -157,6 +191,8 @@ NOTE: The `xpack.fleet.outputs` settings are intended for advanced configuration
     Array that contains the list of host for that output.
   `config`::: 
     Extra config for that output.
+  `proxy_id`:::
+    Unique ID of a proxy to access the output.
 =====
 +
 .Optional properties of `xpack.fleet.outputs`
@@ -167,40 +203,66 @@ NOTE: The `xpack.fleet.outputs` settings are intended for advanced configuration
   `is_default_monitoring`::: 
     If `true`, the output specified in `xpack.fleet.outputs` will be the one used to send agent monitoring data unless there is another one configured specifically for the agent policy.
 =====
+
+`xpack.fleet.fleetServerHosts`::
+List of {fleet-server} hosts that are configured when the {fleet} app starts.
 +
-Example configuration:
+.Required properties of `xpack.fleet.fleetServerHosts`
+[%collapsible%open]
+=====
+  `id`:::
+    Unique ID for the host server.
+  `name`::: 
+    Name of the host server.
+  `host_urls`::: 
+    Array of one or more host URLs that {agents} will use to connect to {fleet-server}.
+=====
 +
-[source,yaml]
-----
-xpack.fleet.packages:
-  - name: apache
-    version: 0.5.0
+.Optional properties of `xpack.fleet.fleetServerHosts`
+[%collapsible%open]
+=====
+  `is_default`:::
+    Whether or not this host should be the default to use for {fleet-server}.
+  `proxy_id`::: 
+    Unique ID of the proxy to access the {fleet-server} host.
+=====
 
-xpack.fleet.agentPolicies:
-  - name: Preconfigured Policy
-    id: 1
-    namespace: test
-    package_policies:
-      - package:
-          name: system
-        name: System Integration
-        id: preconfigured-system
-        inputs:
-          - type: system/metrics
-            enabled: true
-            vars:
-              - name: system.hostfs
-                value: home/test
-            streams:
-              - data_stream:
-                  dataset: system.core
-                enabled: true
-                vars:
-                  - name: period
-                    value: 20s
-          - type: winlog
-            enabled: false
-----
+`xpack.fleet.proxy`::
+List of proxies to access {fleet-server} that are configured when the {fleet} app starts.
++
+.Required properties of `xpack.fleet.proxy`
+[%collapsible%open]
+=====
+  `id`:::
+    Unique ID of the proxy to access the {fleet-server} host.
+  `name`::: 
+    Name of the proxy to access the {fleet-server} host.
+  `url`::: 
+    URL that {agents} use to connect to the proxy to access {fleet-server}.
+=====
++
+.Optional properties of `xpack.fleet.proxy`
+[%collapsible%open]
+=====
+  `proxy_headers`:::
+    Map of headers to use with the proxy.
+.Properties of `proxy_headers`
+[%collapsible%open]
+=======
+  `key`:::: 
+    Key to use for the proxy header.
+  `value`:::: 
+    Value to use for the proxy header.
+=======
+  `certificate_authorities`::: 
+    Certificate authority (CA) used to issue the certificate.
+  `certificate`::: 
+    The name of the certificate used to authenticate the proxy.
+  `certificate_key`::: 
+    The certificate key used to authenticate the proxy.
+=====
 
 `xpack.fleet.enableExperimental`::
-List of experimental feature flag to enable in Fleet.
+List of experimental feature flag to enable in Fleet.
+
+