Update the Metric Threshold rule with the "No data" option

[alaudazzi]

This PR updates the Create a metric threshold rule page with the notion of "No Data" when creating metric threshold alerts.

Closes #4664 (comment).

Generative AI disclosure

1. Did you use a generative AI (GenAI) tool to assist in creating this contribution?

• Yes
• No

[github-actions]

Vale Linting Results

Summary: 3 suggestions found

💡 Suggestions (3)

File	Line	Rule	Message
solutions/observability/incident-management/create-metric-threshold-rule.md	12	Elastic.FirstPerson	Avoid first-person pronouns such as 'me'.
solutions/observability/incident-management/create-metric-threshold-rule.md	71	Elastic.FirstPerson	Avoid first-person pronouns such as 'me'.
solutions/observability/incident-management/create-metric-threshold-rule.md	79	Elastic.FirstPerson	Avoid first-person pronouns such as 'me'.

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[github-actions]

🔍 Preview links for changed docs

• solutions/observability/incident-management/create-metric-threshold-rule.md

[alaudazzi]

@nastasha-solomon @benakansara
I'm still missing a screenshot, but you can already review the doc updates and provide your feedback. Thanks 🙂

[nastasha-solomon]

Left a few comments for your consideration - thank you!