Update 8.0+ "Configure security in Kibana" docs to align with Stack security docs

[jportner]

Meta issue for Stack security: elastic/elasticsearch#80170

Docs links:

• Configure security in Kibana (7.16, 8.0)
• Configure security for the Elastic Stack (7.16, 8.0)

Starting in 8.0, Security will be on by default in Elasticsearch, and Kibana can be configured to connect to Elasticsearch using an enrollment token.
The 8.0 Elastic Stack docs have been updated to instruct users to use Kibana's interactive setup mode to configure Kibana using the enrollment token.

We should update Kibana's docs in 8.0 accordingly. I suggest:

• Change "1. Configure security in Elasticsearch." to point directly to the Stack Security page, and changing the link text to say "Configure Security in Elasticsearch and enroll Kibana"
• Change "2. Configure Kibana to use the appropriate built-in user.", remove this section completely

@lockewritesdocs do you have any opinions?

[elasticmachine]

Pinging @elastic/kibana-security (Team:Security)

[lockewritesdocs]

@jportner, I agree with those changes, but think there's more work to be done than just updating those two steps. For example, we need to update the introduction to explain that security is enabled by default, and that we're writing the configuration to kibana.yml. The links in steps 5 & 6 need to be updated instead of linking to old, redirected pages. Step 8 is no longer required, because users log in as the elastic user when enrolling Kibana. I plan on addressing all of these changes.

Related is the question about what to do with pages like Encrypt communications in Kibana that just point to the Elasticsearch docs. Other pages like Mutual TLS with Elasticsearch should be retained for users who want to configure it, but we should revisit that information and update it.

[jportner]

I opened another issue for follow-on work to restructure the Kibana docs, we'll start by making an outline: #118283