[8.x](backport #4292) Add TheHive connector for cases

[mergify]

Relates to elastic/kibana#180931

Observability cases can use TheHive connector in serverless now and in 8.16 and later releases.
The automated screenshot here was obtained via changes implemented in elastic/kibana#193913

Preview

• https://observability-docs_bk_4292.docs-preview.app.elstc.co/guide/en/observability/master/manage-cases-settings.html#cases-external-connectors
• https://elastic-dot-co-docs-production-a7r625832-elastic-dev.vercel.app/current/serverless/observability/case-settings#external-incident-management-systems

Port

• This PR updates both stateful and serverless content sets

  ---

  This is an automatic backport of pull request Add TheHive connector for cases #4292 done by Mergify.

[mergify]

Cherry-pick of 241b955 has failed:

On branch mergify/bp/8.x/pr-4292
Your branch is up to date with 'origin/8.x'.

You are currently cherry-picking commit 241b955b.
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Changes to be committed:
	modified:   docs/en/observability/manage-cases-settings.asciidoc

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	deleted by us:   docs/en/serverless/cases/manage-cases-settings.mdx
	deleted by us:   docs/en/serverless/images/observability-cases-add-connector.png

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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[bmorelli25]

@Mergifyio update

[mergify]

update

☑️ Nothing to do

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