[8.15](backport #4113) Port EEM and new services experience docs to stateful

[mergify]

Ports #4062 to serverless. Also makes some minor tweaks to serverless docs to mirror structural changes needed for stateful docs.

Previews:

• New services experience
• Elastic Entity Model

TODO:

• Add topics to the correct place in the nav after Move APM UI documentation to observability-docs with history #3723 is merged.
• Finish porting all changes
• Update link when topic about defining service.name has been ported to stateful (requires Port logs service.name docs to stateful #4116)

---

This is an automatic backport of pull request #4113 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 618cc64 has failed:

On branch mergify/bp/8.15/pr-4113
Your branch is up to date with 'origin/8.15'.

You are currently cherry-picking commit 618cc64b.
  (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/observability/apm-ui/index.asciidoc
	new file:   docs/en/observability/apm-ui/new-experience-services.asciidoc
	modified:   docs/en/observability/apm-ui/services.asciidoc
	new file:   docs/en/observability/apm/images/entity-centric-services-inventory.png
	new file:   docs/en/observability/elastic-entity-model.asciidoc
	modified:   docs/en/observability/index.asciidoc
	new file:   docs/en/observability/reference.asciidoc

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	deleted by us:   docs/en/serverless/elastic-entity-model.mdx

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

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[dedemorton]

LGTM

[dedemorton]

Conflicts were in serverless folder, which has been deleted for the backport, so the conflict noted in this PR is not relevant.