[8.x] [Request][Serverless][8.16] Document the new kibana.alert.rule.execution.type field being added for manual runs (backport #5940)

[mergify]

Fixes #5922 by doing the following:

• Doc'd the new kibana.alert.rule.execution.type alert field
• Updated the description for the kibana.alert.intended_timestamp field, which now shows the same value as the @timstamp field for alerts generated by scheduled runs

Previews:

• ESS: https://security-docs_bk_5940.docs-preview.app.elstc.co/guide/en/security/master/alert-schema.html
• Serverless: https://elastic-dot-co-docs-production-c9b4zfjhy-elastic-dev.vercel.app/current/serverless/security/alert-schema

  ---

  This is an automatic backport of pull request [Request][Serverless][8.16] Document the new kibana.alert.rule.execution.type field being added for manual runs #5940 done by Mergify.

[mergify]

Cherry-pick of c73c723 has failed:

On branch mergify/bp/8.x/pr-5940
Your branch is up to date with 'origin/8.x'.

You are currently cherry-picking commit c73c723f.
  (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/reference/alert-schema.asciidoc

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	deleted by us:   docs/serverless/alerts/alert-schema.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

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