[DOCS] Fleet and Endpoint Security require a Transform, appropriate configuration is needed

[kevinlog]

Description

There have been some sdh issues that have come in where users have custom node and cluster configurations which disallow transforms from running. Right now, this is a problem because Endpoint security requires a transform to operate and since Endpoint is a required package, Fleet also requires that this transform is installed properly. I will prioritize that we fix this situation before the end of 7.last, however for the time being, we should make this restraint more clear in documentation.

We should list a set of requirements for the Security solution to ensure that users have their node roles and clusters configured correctly in order to use Endpoint security.

Node roles

Users must have at least one node have a transform role.

By default, node roles will include this role. As soon as a user customizes their roles, they will need to ensure that at least one has the transform role.

More information here on Node roles: https://www.elastic.co/guide/en/elasticsearch/reference/current/modules-node.html

Cluster settings

Users must have their Cluster settings setup so that it will allow persistent tasks.

By default, the Cluster settings will allow these persistent tasks, however if users customize them, it is possible to turn it off.

If the user runs GET /_cluster/settings in dev tools and finds the below setting, transforms will not be allowed to install:

{
  "transient": {
    "cluster.persistent_tasks.allocation.enable": "none"
  }
}

They will need to ensure this is set to (the default setting):

{
  "transient": {
    "cluster.persistent_tasks.allocation.enable": "all"
  }
}

More information on Cluster settings: https://www.elastic.co/guide/en/elasticsearch/reference/current/cluster-update-settings.html
relevant SDH: https://github.com/elastic/sdh-kibana/issues/1398

Acceptance Test Criteria

• As a user, I should ensure that I have at least one node.role with a transform role
• As a user, I should ensure that my cluster settings allow for persistent_tasks allocation

Notes

• Add the "Team:Docs" label to new issues.
• Be sure to add the version number label.
• Be sure to add any necessary screenshots for clarity.
• Include any conditions or caveats that may affect customers.

[jmikell821]

Merged #693.