[8.x](backport #4345) [synthetics] Update monitor status rule docs

[mergify]

Description

The Synthetics monitor status rule type was improved in elastic/kibana#186585. This PR makes a few updates to the Create a monitor status rule doc including:

• Marking the Uptime rule as deprecated and making it clear that the Synthetics rule is the right choice moving forward.
• Fully documenting the Synthetics monitor status rule type with details on the available filters, conditions, action types, and action variables.
• Documenting how users who have an existing Uptime rule that they are using for their Synthetics monitors can replace the Uptime rule with the Synthetics rule.
  • @shahzad31 @paulb-elastic I'm not sure if this section (preview) is what you had in mind. I'm not sure there's a 1:1 comparison for everything included in the Uptime rule unless I'm misunderstanding. For example, I don't think you can recreate the Uptime rule's availability condition exactly using the new Synthetics rule conditions (would you use a synthetics SLI instead?). Let me know if I'm missing something or if there's a different way we should approach this.

Note: I documented the Uptime and Synthetics rules on the same page titled "Create a monitor status rule" because the "Alerts and rules" dropdown in both the Uptime app and Synthetics UI uses the language "Monitor status rule", but I could be persuaded to document them separately. Either way, starting in 9.0.0, I imagine the Uptime content on this page will go away.

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 #4164
Closes #4337

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 #4345 done by [Mergify](https://mergify.com).

[mergify]

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Changes to be committed:
	new file:   docs/en/observability/images/synthetic-monitor-action-types-each-alert.png
	new file:   docs/en/observability/images/synthetic-monitor-action-types-more-options.png
	new file:   docs/en/observability/images/synthetic-monitor-action-types-summary.png
	new file:   docs/en/observability/images/synthetic-monitor-action-variables.png
	new file:   docs/en/observability/images/synthetic-monitor-conditions.png
	new file:   docs/en/observability/images/synthetic-monitor-filters.png
	modified:   docs/en/observability/monitor-status-alert.asciidoc
	new file:   docs/en/serverless/alerting/synthetic-monitor-status-alert.mdx
	new file:   docs/en/serverless/images/synthetic-monitor-action-types-each-alert.png
	new file:   docs/en/serverless/images/synthetic-monitor-action-types-more-options.png
	new file:   docs/en/serverless/images/synthetic-monitor-action-types-summary.png
	new file:   docs/en/serverless/images/synthetic-monitor-action-variables.png
	new file:   docs/en/serverless/images/synthetic-monitor-conditions.png
	new file:   docs/en/serverless/images/synthetic-monitor-filters.png

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	deleted by us:   docs/en/serverless/serverless-observability.docnav.json

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

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