Update/consolidate Fleet kibana.yml settings

[jen-huang]

There is a page for xpack.fleet.* settings here: https://www.elastic.co/guide/en/kibana/current/fleet-settings-kb.html

But it is now outdated. I also thought it was odd to have this away from the rest of the Fleet docs, like https://www.elastic.co/guide/en/fleet/current/fleet-settings.html.

Here's a list of valid settings:
xpack.fleet.enabled: see Kibana settings doc
xpack.fleet.registryUrl: see Kibana settings doc
xpack.fleet.registryProxyUrl: see elastic/kibana#78648
xpack.fleet.agents.enabled: enable or disable central management of agents
xpack.fleet.agents.elasticsearch.hosts
xpack.fleet.agents.elasticsearch.ca_sha256
xpack.fleet.packages
xpack.fleet.agentPolicies

[Zacqary]

This PR contains usage examples of packages and agentPolicies: elastic/kibana#96588

packages

This is a list of integration packages to be installed when the Fleet app starts up for the first time. Required props are:

• name: the name of the package
• version: either an exact semver, or the keyword latest to fetch the latest package version

agentPolicies

This is a list of agent policies to configure when the Fleet app starts. Required properties are:

• name: a name of the policy
• id: a number or string acting as a unique ID for this policy

Optional properties are:

• description: A text description of this policy
• namespace: a string identifying this policy's namespace, for advanced use cases
• monitoring_enabled: A list of either or both the keywords logs or metrics
• is_managed: if true, this policy will not be editable by the user, and can only be changed by updating the Kibana config
• is_default: if true, this will be considered the default agent policy
• is_default_fleet_server: if true, this will be considered the default agent policy for the Fleet Server
• package_policies: A list of policy integrations to add to this policy
  • name: (required) the name of the integration policy
  • package: (required) An object containing
    • name: the name of the integration package associated with this policy
  • description: a text string describing this integration policy
  • namespace: a string identifying this policy's namespace
  • inputs: An array that overrides any default input settings for this policy's package. 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.