[OBSERVABILITY][ResponseOps][Alerting][8.19] Alert Deletion Docs

[nastasha-solomon]

⚠️ NOTE: Must be merged after elastic/kibana#229230. ⚠️

Description

Added a new section about cleaning up old alerts. Also expanded the page's title.

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 elastic/docs-content#1716

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: [DOCS][ResponseOps][Alerting][8.19] Alert Deletion Docs  kibana#229230 and [SECURITY][ResponseOps][Alerting][8.19] Alert Deletion Docs security-docs#6952
  • Port to serverless docs: [ResponseOps][Alerting][9.1 & Serverless] Alert Deletion Docs docs-content#2138

[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.}

[nastasha-solomon]

The CI checks for this PR will fail until elastic/kibana#229230 is merged.

[nastasha-solomon]

run docs-build

[nastasha-solomon]

run docs-build rebuild