[8.x](backport #4488) Quickstart k8s otel doc added

[mergify]

Description

Adding a quickstart document for Unified Kubernetes Observability with EDOT.

Preview: https://observability-docs_bk_4488.docs-preview.app.elstc.co/guide/en/observability/master/monitor-k8s-otel-edot.html

The objective of this doc is to be a good entry point for this use case, considering that we have all details and detailed documents in elastic/opentelemetry repo: https://github.com/elastic/opentelemetry/tree/main/docs/kubernetes/operator

If possible I'd like to get a review by @mdbirnstiehl or @bmorelli25 before requesting the review by @mlunadia and @akhileshpok .

Initial concerns:

• The pre-requisites section looks a bit ugly and not very homogeneous.
• The title is a bit long, but I think this is what Miguel or Akhilesh wanted (we can check with them at a later stage).
• Should we add a similar doc to Serverless?

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 #4428

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 #4488 done by [Mergify](https://mergify.com).

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