Add docs and edit strings for Beats Central Management

[dedemorton]

We need to update the documentation for the new Beats Central Management feature: #7028

To do list for beta:

• Review UI text strings and messages.
  • Make text strings clear, concise, and consistent [Gail]
  • Suggest changes to improve usability and general user experience for Beats users [DeDe]
• Add topic about Beats Central Management to the Kibana docs (high-level overview content parallel to what we provide for similar UIs). [DeDe]
• Add topic(s) about Beats Central Management to the Beats docs (deep dive on concepts and tasks required to use the UI). [DeDe]
• Update getting started and reference content about configuration to include links to the docs about central management [DeDe]
• Removed from this list; now tracked in Restructuring: Improve reference content about configuration options #9363. Improve docs about config settings to make sure they play well with the new UI (best effort, stretch goal). [DeDe] [removing because this is a longer term goal]
• Update security docs to show requirement for beats_admin role. [DeDe]
• Removed from this list; now tracked in Document API for enrolling beats in central management #9001: Document public API (best effort). First effort might be manual, but we would like to generate these docs eventually.

[exekias]

I've started a new branch to work on initial docs in the beats side: https://github.com/elastic/beats/compare/master...exekias:cm-docs?expand=1

[dedemorton]

PR with the doc additions is here: #9054

[dedemorton]

Closing this issue. The remaining items are tracked in issue #9363 and #9001.