[8.15](backport #4346) [synthetics] Document Synthetics availability SLI

[mergify]

Description

Document the new Synthetics availability SLI.

Preview: Create a service-level objective (SLO) > Synthetics availability

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 #4299

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 #4346 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of c33dd95 has failed:

On branch mergify/bp/8.15/pr-4346
Your branch is up to date with 'origin/8.15'.

You are currently cherry-picking commit c33dd958.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   docs/en/observability/slo-create.asciidoc
	modified:   docs/en/observability/synthetics-configuration.asciidoc

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	deleted by us:   docs/en/serverless/slos/create-an-slo.mdx
	deleted by us:   docs/en/serverless/transclusion/synthetics/configuration/monitor-config-options.mdx

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

[github-actions]

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