[8.14](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

---

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

[mergify]

Cherry-pick of d6a61fd has failed:

On branch mergify/bp/8.14/pr-4193
Your branch is up to date with 'origin/8.14'.

You are currently cherry-picking commit d6a61fd1.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   docs/en/apm-server/redirects.asciidoc
	modified:   docs/en/observability/apm/command-reference.asciidoc
	modified:   docs/en/observability/redirects.asciidoc

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both added:      docs/en/observability/apm/feature-roles.asciidoc

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[github-actions]

A documentation preview will be available soon.

• 🔨 Buildkite builds
• 📚 HTML diff
• 📙 Preview page

Request a new doc build by commenting

• Rebuild this PR: run docs-build
• Rebuild this PR and all Elastic docs: run docs-build rebuild

_{run docs-build is much faster than run docs-build rebuild. A rebuild should only be needed in rare situations.}

_{If your PR continues to fail for an unknown reason, the doc build pipeline may be broken. Elastic employees can check the pipeline status here.}