[Fleet] Add auto-upgrade settings to documentation (elastic#239448)

## Summary

Relates to elastic/cloud#147631.

Document `xpack.fleet.autoUpgrades.*` settings now being available to
configure for ECH per elastic/cloud#148147.

They were in product docs already here:
https://www.elastic.co/docs/reference/fleet/upgrade-elastic-agent#auto-upgrade-settings

Also saw that the settings were missing on this page so added them there
too:
https://www.elastic.co/docs/reference/kibana/configuration-reference/fleet-settings

### Checklist
- [x]
[Documentation](https://www.elastic.co/guide/en/kibana/master/development-documentation.html)
was added for features that require explanation or tutorials
- [x] Review the [backport
guidelines](https://docs.google.com/document/d/1VyN5k91e5OVumlc0Gb9RPa3h1ewuPE705nRtioPiTvY/edit?usp=sharing)
and apply applicable `backport:*` labels.

---------

Co-authored-by: Visha Angelova <[email protected]>

Author: jen-huang

docs/reference/cloud/elastic-cloud-kibana-settings.md

@@ -108,6 +108,12 @@ stack: ga 9.1
 `xpack.product_intercept.interval`:
 :   Set the time that elapses between Elastic product feedback prompts. The time is formatted as a number and a time unit (d,h,m,s). For example, 20m, 24h, 7d. Defaults to `90d`.
 
+`xpack.fleet.autoUpgrades.taskInterval`:
+:   Configure the interval of the automatic upgrade task for {{fleet}}-managed {{agents}}. Defaults to `30m`.
+
+`xpack.fleet.autoUpgrades.retryDelays`:
+:   Configure the retry delays of the automatic upgrade task for {{fleet}}-managed {{agents}}. The array's length indicates the maximum number of retries. Defaults to `['30m', '1h', '2h', '4h', '8h', '16h', '24h']`.
+
 ### Version 8.18+ [ec_version_8_18]
 
 `xpack.fleet.enableManagedLogsAndMetricsDataviews`

docs/reference/configuration-reference/fleet-settings.md

@@ -344,7 +344,11 @@ These settings are not supported to pre-configure the Endpoint and Cloud Securit
     Experimental features should not be enabled in production environments. The features in this section are experimental and may be changed or removed completely in future releases. Elastic will make a best effort to fix any issues, but experimental features are not supported to the same level as generally available (GA) features.
     ::::
 
-`xpack.fleet.enableManagedLogsAndMetricsDataviews`
+`xpack.fleet.enableManagedLogsAndMetricsDataviews` ![logo cloud](https://doc-icons.s3.us-east-2.amazonaws.com/logo_cloud.svg "Supported on {{ech}}")
 :   Set to `true` (default), to enable the automatic creation of global `logs-*` and `metrics-*` data views.
 
+`xpack.fleet.autoUpgrades.taskInterval` ![logo cloud](https://doc-icons.s3.us-east-2.amazonaws.com/logo_cloud.svg "Supported on {{ech}}") {applies_to}`stack: ga 9.1`
+:   Configure the interval of the automatic upgrade task for {{fleet}}-managed {{agents}}. Defaults to `30m`.
 
+`xpack.fleet.autoUpgrades.retryDelays` ![logo cloud](https://doc-icons.s3.us-east-2.amazonaws.com/logo_cloud.svg "Supported on {{ech}}") {applies_to}`stack: ga 9.1`
+:   Configure the retry delays of the automatic upgrade task for {{fleet}}-managed {{agents}}. The array's length indicates the maximum number of retries. Defaults to `['30m', '1h', '2h', '4h', '8h', '16h', '24h']`.