[8.x](backport #4486) Adding SLOs troubleshooting document (attempt 2)

[mergify]

Description

New document Troubleshoot SLOs created, per #4237 request.
SLO pre-requisites updated in all pages
SLO Beta instructions moved to troubleshooting doc

Preview:

• https://observability-docs_bk_4486.docs-preview.app.elstc.co/guide/en/observability/master/slo-troubleshoot-slos.html
• https://observability-docs_bk_4486.docs-preview.app.elstc.co/guide/en/observability/master/slo.html

Before moving forward I'd like @lucabelluccini to take a look at the current structure and decide if all suggested parts are useful.

After agreeing on main structure and areas to talk about we can discuss further the exact content.

Pending actions

• Analyze and decide if serverless documentation needs to be updated.

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

Closes #4237

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 #4486 done by [Mergify](https://mergify.com).

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