[8.x] [Request] [8.18, 9.0, and Serverless] Observables can be added to cases (backport #6477)

[mergify]

Fixes #6395

ESS:

• Open and manage cases | Add observables: New section that explains how to add observables and view similar cases
• Configure case settings | Observable types: New section that explains how to create custom observable types

Serverless:

• Create and manage cases | Add observables
• Configure case settings | Observable types

---

This is an automatic backport of pull request #6477 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 966c104 has failed:

On branch mergify/bp/8.x/pr-6477
Your branch is up to date with 'origin/8.x'.

You are currently cherry-picking commit 966c1044.
  (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-settings.asciidoc
	modified:   docs/cases/cases-manage.asciidoc
	new file:   docs/cases/images/cases-add-observables.png
	modified:   docs/cases/images/cases-add-template.png
	modified:   docs/cases/images/cases-home-page.png
	modified:   docs/cases/images/cases-manage-comments.png
	new file:   docs/cases/images/cases-observable-types.png
	modified:   docs/cases/images/cases-settings.png
	modified:   docs/cases/images/cases-summary.png
	modified:   docs/cases/images/cases-ui-open.png
	new file:   docs/serverless/images/cases-settings/security-cases-observable-types.png

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	deleted by us:   docs/serverless/images/cases-open-manage/-cases-cases-home-page.png
	deleted by us:   docs/serverless/images/cases-open-manage/-cases-cases-manage-comments.png
	deleted by us:   docs/serverless/images/cases-open-manage/-cases-cases-summary.png
	deleted by us:   docs/serverless/images/cases-open-manage/-cases-cases-ui-open.png
	deleted by us:   docs/serverless/images/cases-settings/security-cases-settings.png
	deleted by us:   docs/serverless/images/cases-settings/security-cases-templates.png
	deleted by us:   docs/serverless/investigate/cases-open-manage.asciidoc
	deleted by us:   docs/serverless/investigate/cases-settings.asciidoc
	added by them:   docs/whats-new/images/8.7/-cases-cases-add-observables.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

[github-actions]

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