[8.x](backport #4628) Add note about quickstart to logs stream docs

[mergify]

Description

This PR clarifies that the Logs stream docs show how to manually configure a standalone agent, and users also have the option of using the automatic log detection in the quickstart.

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

Checklist

• Product/Engineering Review
• Writer Review

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 is an automatic backport of pull request #4628 done by [Mergify](https://mergify.com).

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