[8.x](backport #4872) Hide EEM and inventory pages

[mergify]

Description

Hides the EEM and Inventory pages from appearing in the table of contents and from search engines. However, the pages will still be published in case any users have explicitly enabled entityCentricExperience.

Documentation sets edited in this PR

Check all that apply.

• Stateful (docs/en/observability/*)
• Serverless (docs/en/serverless/*)
• Integrations Developer Guide (docs/en/integrations/*)
• None of the above

Related issue

Related to https://github.com/elastic/observability-docs/issues/4690, elastic/docs-content#1057

Checklist

• Product/Engineering Review
• Writer Review

Follow-up tasks

Select one.

• This PR does not need to be ported to another doc set because:
  • The concepts in this PR only apply to one doc set (serverless or stateful)
  • The PR contains edits to both doc sets (serverless and stateful)
• This PR needs to be ported to another doc set:
  • Port to stateful docs: <link to PR or tracking issue>
  • Port to serverless docs: <link to PR or tracking issue>

---

This is an automatic backport of pull request #4872 done by [Mergify](https://mergify.com).

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