[Serverless bug bash] Updates cases API link (backport #6160)

[mergify]

Serverless bug bash

Resolves #6016 by updating the cases API links to lead to the API reference site.

The same fix has been applied to ESS docs.

Previews

Serverless:

• Cases
• Create and manage cases
• Add alerts to cases

ESS:

• Cases
• Open and manage cases
• Add detection alerts to cases

---

This is an automatic backport of pull request #6160 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 115cbe6 has failed:

On branch mergify/bp/8.17/pr-6160
Your branch is up to date with 'origin/8.17'.

You are currently cherry-picking commit 115cbe6e.
  (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/cases/cases-manage.asciidoc
	modified:   docs/cases/cases-overview.asciidoc
	modified:   docs/detections/alerts-add-to-cases.asciidoc

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	deleted by us:   docs/serverless/alerts/signals-to-cases.asciidoc
	deleted by us:   docs/serverless/investigate/cases-open-manage.asciidoc
	deleted by us:   docs/serverless/investigate/cases-overview.asciidoc

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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