[8.15](backport #4301) Update alert history chart screenshots

[mergify]

Description

Update alert history chart screenshots in the Triage SLO burn rate breaches (preview) and Triage threshold breaches (preview) pages.

This PR also makes a few other changes:

• Splits screenshots into individual images for each chart to make it easier to maintain.
• Attempts to make the list describing charts more browsable with bold text at the beginning.
• Moves the last bullet point about timeline annotations to the first bullet point on both pages because (I think) with the change to the alert history chart, there are no longer annotations in alerts history.

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 #4129

Checklist

• Product/Engineering Review
• Writer Review
  (@dedemorton I'd be particularly interested in your opinion of the changes since I think you wrote the initial content)

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: Port #4301 to serverless #4320

---

This is an automatic backport of pull request #4301 done by [Mergify](https://mergify.com).

[github-actions]

A documentation preview will be available soon.

• 🔨 Buildkite builds
• 📚 HTML diff
• 📙 Preview page

Request a new doc build by commenting

• Rebuild this PR: run docs-build
• Rebuild this PR and all Elastic docs: run docs-build rebuild

_{run docs-build is much faster than run docs-build rebuild. A rebuild should only be needed in rare situations.}

_{If your PR continues to fail for an unknown reason, the doc build pipeline may be broken. Elastic employees can check the pipeline status here.}

[colleenmcginnis]

🐣