Document no data settings for custom threshold rule

[dedemorton]

Description

Adds documentation that explain new setting "no data" setting for the custom threshold rule.

Link to preview: https://observability-docs_bk_4418.docs-preview.app.elstc.co/guide/en/observability/master/custom-threshold-alert.html#trigger-alert-when-no-data

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 #4068

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: ADD ISSUE LINK BEFORE MERGING

[github-actions]

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[mergify]

This pull request does not have a backport label. Could you fix it @dedemorton? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-/d./d is the label to automatically backport to the /d./d branch. /d is the digit
  NOTE: backport-skip has been added to this pull request.

[bmorelli25]

run docs-build

[maryam-saeidi]

That's amazing! Thanks! 💖

Does it make sense also to mention that "no data" has its own action group? Is this already covered somewhere?

[dedemorton]

Does it make sense also to mention that "no data" has its own action group? Is this already covered somewhere?

That's covered where actions are described, so I don't think I would mention it here.

[eedugon]

Looks great to me ;)