[8.x](backport #4193) [apm] Update APM feature roles docs

[mergify]

In #3980 (comment), @endorama suggested that we clarify the docs on APM feature roles. Let me know what you think of this approach. I attempted to clarify how feature roles work and which privileges are required for different use cases by:

• Reframing the intro to feature roles to emphasize that the what we're recommending is to create several feature-related roles and then assign one or more of these roles to each user (or group) based on which features the user needs to access.
• Providing an example of when a user might need to be assigned multiple roles from this list of feature roles (this needs work).
• Emphasizing that the Central configuration management role is required in most cases (unless central configuration management is explicitly disabled by the org/user).
• Putting all roles on a single page to increase the likelihood that the reader will see the context provided in the intro paragraphs.
• Adding a step at the end of the instructions for creating each role to remind users that they can assign other APM feature roles to the users as needed.

Here's what hasn't been addressed yet:

• Verify that the privileges listed in each role are correct and reflect the latest version of APM / Elastic as a whole. (I'm not sure how to go about confirming or testing these.)
• Are these the right feature roles? Are any missing? Should any be removed?

Here's the PR preview: https://observability-docs_bk_4193.docs-preview.app.elstc.co/guide/en/observability/master/apm-feature-roles.html

@endorama let me know what you think of this approach. If we're on the right track, we can iterate in this PR.

cc @simitt @carsonip

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This is an automatic backport of pull request #4193 done by Mergify.

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